Hi Ruben,
I wanted to give you a new possibility with a little change to inventory.c. I added a new occurrence to the slot click event in order to detect the mouse right button.

You need to add a new macro for the new occurrence
Code:
#define INV_ITEM_RIGHT_CLICK 4 // 354th code line


The modified slotOnClick function: the function added as event to every slot.
Code:
function slotOnClick(PANEL* clicked_panel) 
{
	Slot* clicked_slot = getSlotPtrForPanelPtr(clicked_panel);

	if (clicked_slot == NULL) { diag("\n*** Error: panel not found\n"); }
	
	if (global_floating_item_ptr->item == NULL) 	// player clicked on item in inventory while not floating an item
	{
// 16/04/2014 Addition -----------------------------------------------------
		if ( event_type == EVENT_CLICK ) // mouse left button event
		{
// -------------------------------------------------------------------------
			if (clicked_slot->item != NULL) // Player is grabbing an item out of the inventory
			{
				// optionally callback with action, bag_id, slot_id, item_id
				if (on_click_callback_function_ptr) 
					on_click_callback_function_ptr(INV_ITEM_REMOVED,clicked_slot->bag_id,clicked_slot->id,0,clicked_slot->item->id);
	
	
				// Create the floating panel
				
				global_floating_item_ptr->floating_item_panel = pan_create("",floating_item_layer);
				
				// Set the item and panel image of the floating item structure.  This effectively "floats"
				// the item that was stored in the inventory slot.
				
				global_floating_item_ptr->item = clicked_slot->item;
				global_floating_item_ptr->floating_item_panel->bmap = bmap_create(clicked_slot->item->floating_icon);
				global_floating_item_ptr->floating_item_panel->flags |= VISIBLE;
				
				// Set the item pointer of the clicked on slot to NULL
				
				clicked_slot->item = NULL;
				
				// Restore inventory slot image to be the default background image
				
				clicked_slot->slot_panel->bmap = bmap_create(clicked_slot->background_image);			
	
				inv_floating_flag = 1;
				_inv_do_float(global_floating_item_ptr);
				
			}
// 16/04/2014 Addition -----------------------------------------------------
		}
		else if ( event_type == EVENT_RIGHTCLICK ) // mouse right button event
		{
			if (clicked_slot->item != NULL)
			{
				if (on_click_callback_function_ptr) 
					on_click_callback_function_ptr(INV_ITEM_RIGHT_CLICK,clicked_slot->bag_id,clicked_slot->id,clicked_slot->item->id,0);
			}
		}
// -------------------------------------------------------------------------
	}
	else
	{
		if ((clicked_slot->group_id == 0) || (clicked_slot->group_id == global_floating_item_ptr->item->group_id)) // Test to make sure that item can be placed in the slot
		{		
			if (clicked_slot->item == NULL) // player is putting an item into an empty inventory slot
			{
				// optionally callback with action, bag_id, slot_id, item_id
				if (on_click_callback_function_ptr) on_click_callback_function_ptr(INV_ITEM_PLACED,clicked_slot->bag_id,clicked_slot->id,global_floating_item_ptr->item->id,0);
				
				// Place item in slot
				clicked_slot->item = global_floating_item_ptr->item;	
				clicked_panel->bmap = bmap_create(global_floating_item_ptr->item->inventory_icon);
				global_floating_item_ptr->item = NULL;		
	
				// End float
				pan_remove(global_floating_item_ptr->floating_item_panel);
				inv_floating_flag = 0;	
			}		
			else // player is swapping the floating item for the one in the inventory
			{
	
				// optionally callback with action, bag_id, slot_id, item_id
				if (on_click_callback_function_ptr) on_click_callback_function_ptr(INV_ITEM_SWAPPED,clicked_slot->bag_id,clicked_slot->id,global_floating_item_ptr->item->id,clicked_slot->item->id);
				
				
				// Backup the information of the clicked on slot
				Item* clicked_slot_item_pointer = clicked_slot->item;
				STRING* clicked_slot_inventory_icon = str_create(clicked_slot->item->inventory_icon);
				
				// Set the slot to the floating item
				clicked_slot->item = global_floating_item_ptr->item;
				clicked_panel->bmap = bmap_create(global_floating_item_ptr->item->inventory_icon);
				
				// Set the floating item to the slot
				global_floating_item_ptr->item = clicked_slot_item_pointer;
				global_floating_item_ptr->floating_item_panel->bmap = bmap_create(clicked_slot_inventory_icon);
				
			}
		}
	}
}


Since every slot is a panel, they are not buttons, you must set enable_mouse to 2 in order to get the event call when you click with the right mouse button over a slot.

Code:
mouse_mode = 2; // or 4 or whatever you use
enable_mouse = 2;


At this point you should be able to detect the mouse right click over a slot inside your slot_was_clicked function.

Code:
function slot_was_clicked ( int occurrence, int bag_id, int slot_id, int placed_item_id, int removed_item_id ) 
{
	Bag *bagPanel;
	
	if ( ( occurrence == INV_ITEM_REMOVED ) || ( occurrence == INV_ITEM_SWAPPED ) )
	{
		if (bag_id == MAIN_BAG_ID)
		{
			if (slot_id == ACTIV_WPN_SLOT_ID)
			{
				ent_remove ( active_weapon );
				
				if(placed_item_id == NULL)
				{
					weapon_code = fist_code;
				}
			}
		}   
	}
	if ( ( occurrence == INV_ITEM_PLACED ) || ( occurrence == INV_ITEM_SWAPPED ) )
	{
		if (bag_id == MAIN_BAG_ID)
		{
			if (slot_id == ACTIV_WPN_SLOT_ID)
			{
				switch ( placed_item_id )
				{
					case SWORD:   activate_sword();  break;
					case MACE:    activate_mace();  break; 
					default:      weapon_code = fist_code; break;
				}
			}
		}   
	}
	if ( occurrence == INV_ITEM_RIGHT_CLICK )
	{
		snd_play ( sndExploDebug, 100, 0 );
	}
}


This code let you hear the sndExploDebug sound when you click over a filled slot and when you have not an item grabbed.

Imagine you want to drop an item at the player position when you click over a filled slot with the mouse right button. You would need to create a new entity in the scenery and clear the clicked slot. You know how to create a new entity but there is no function to clear a slot. I wrote it for you. Add it to inventory.c and use it into your slot_was_clicked function.

Code:
function inv_clear_slot ( int bag_id, int slot_id )
{
	Bag* bag;
	
	int ii;
	for ( ii=0; ii<inv_bag_count; ii++ )
	{
		bag = inv_bags_array[ii];
		if ( bag->id != bag_id )
			continue;
		
		int i = 0;
		for ( ; i<bag->_slot_count; i++ )
		{
			Slot *slot = bag->slots[i];
			if ( slot->id != slot_id )
				continue;
			
			if ( slot->item == NULL )
				continue;
			
			if (on_click_callback_function_ptr)
				on_click_callback_function_ptr(INV_ITEM_REMOVED,bag_id,slot_id,slot->item->id,slot->item->id);
			
			slot->item = NULL;
			if ( slot->slot_panel != NULL )
				slot->slot_panel->bmap = bmap_create(slot->background_image);
		}
	}
}


This function gets slower the more bags exists but the item index management does not let me build it other way.

Hope it helps
Salud!