It seems that the mesh data is stored as *.b3d - files (Blitz Basic 3D model format). So own trunks, branches and leave meshes (used in that painter tool) should be possible since TMG3 parses the directory - I added and used my own leaf images in combination with the painter - tool and it has been automatically being recognized, so I guess this works with own meshes as well, though, I didnt tested it so far.

BTW: Milkshape support b3d export

Cheers!