Ah, yes I am used to GLSL providing my inputs for me. 
I gave it a try but it seems to behave the same as gl_Vertex.
I suspect the data provided by gl_Vertex (and p3d_Vertex) is correct, since objects with a default transform seem to show up with their points in the right place.
Perhaps gl_ModelViewProjectionMatrix is getting the wrong transform, or the object is being culled out entirely.
I tested this with the same code as before, I’ll give the new code a try and see if that fixed anything.