Here is some code I have used to sew terrains together that works well.
One function sews top/bottom and the other sews left/right. The code is
pretty self explanatory. I used this on MDLs not HMPs, but I dont think
it makes a difference. I hope this helps someone

Code:

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//two functions to sew seams together in MDL tiles
//models must not have the same file name or be duplicates!
//models must have the same number of verts along each side to be sewn
//the #1 vert is upper right in med, lower left in 3dsmax
//set the matrix variable in each sew function to the number of verts along a side.
//
//


function sew_top_to_bottom(ent1,ent2)    // sew top of ent1 to the bottom of ent2  
{
  my = ent1;
  you = ent2;

  var matrix = 51;   //<< set this to the amount of vertices along your models side  
  var vpos;
  var vpos2;
  var i=0;

  while(i<matrix)
   {
      i+=1;
      vec_for_mesh(vpos,my,i);  //my top row  
      vec_for_mesh(vpos2,you,matrix*(matrix-1) + i);//your bottom row
      vpos.z += (vpos2.z + you.z/you.scale_z) - (vpos.z + my.z/my.scale_z);
      vpos.x += (vpos2.x + you.x/you.scale_x) - (vpos.x + my.x/my.scale_x);
      vpos.y += (vpos2.y + you.y/you.scale_y) - (vpos.y + my.y/my.scale_y);
      vec_to_mesh(vpos,my,i);  //move my top row vert 		
   }
} 

//   
function sew_left_to_right(ent1,ent2) // sew left of ent1 to the right of ent2  
{
  my = ent1;
  you = ent2;

  var matrix = 51; //<< set this to the amount of vertices along your models side    
  var vpos;
  var vpos2;
  var i=0;

  while(i<matrix)
   {
      i+=1;
      vec_for_mesh(vpos,my,matrix*i-(matrix-1));  //my left edge    
      vec_for_mesh(vpos2,you,matrix*i);  //your right edge 
      vpos.z += (vpos2.z + you.z/you.scale_z) - (vpos.z + my.z/my.scale_z);
      vpos.x += (vpos2.x + you.x/you.scale_x) - (vpos.x + my.x/my.scale_x);
      vpos.y += (vpos2.y + you.y/you.scale_y) - (vpos.y + my.y/my.scale_y);
      vec_to_mesh(vpos,my,matrix*i-(matrix-1)); //move my left edge vert 		
   }
} 
  
// Assuming there are nine entity tiles to be sewn together, named terrain1 to terrain9
// that are arranged 3x3 from left to right;
//
//  1 2 3
//  4 5 6
//  7 8 9
//
// we can sew them all together like this;
//


function init_terrain()
{
   sew_top_to_bottom(terrain4,terrain1);  //sew top of ent1 to bottom of ent2   
   sew_top_to_bottom(terrain5,terrain2);
   sew_top_to_bottom(terrain6,terrain3);
   sew_top_to_bottom(terrain7,terrain4);
   sew_top_to_bottom(terrain8,terrain5);
   sew_top_to_bottom(terrain9,terrain6);
   //
   sew_left_to_right(terrain2,terrain1);   //sew left side of ent1 to right side of ent2   
   sew_left_to_right(terrain3,terrain2);
   sew_left_to_right(terrain5,terrain4);
   sew_left_to_right(terrain6,terrain5);
   sew_left_to_right(terrain8,terrain7);
   sew_left_to_right(terrain9,terrain8);
}
 

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