Here is an idea how you could do this in gamestudio:
Create a 2nd view below the camera (layer = -1), make it smaller than the camera (depending on how pixelated you want the entities). Make the 2nd view not render entities with flag1 set (there is a flag in the viewfor that..). Set all the normal (non-pixelated) entities flag1, so they will not be rendered in the 2nd view.
Now you have one normal view (the camera), and a small view with the (not yet) pixelated entities. Now you use the 2nd views .bmap (pro feature) as the skin of a view entity that is above the camera. Give the view entity a material that renders it using a point sampler and makes all the parts where it is black (because nothing was rendered there), transparent.