Thanks for the information.
I've found out that most .obj files will 'open' natively in TGTP (but cannot be rendered from there - they need to be saved as .tgo files, then opened, which means you can by pass Wings3D altogether).
But a few .obj files will crash TGTP just by trying to load them. In such a case, you'll have to save it back and forth between .obj and .lwo in Wings3D (which I assume 'strips' the file of whatever is causing TGTP to crash).
Once you get the .obj files into TGTP as .tgo files, you'll still need to manually set them up with their colors or textures. This can be a challenge at times, but most of the time, if they already have a texture (with alpha), it works like a charm. This works actually well for lower res weeds that don't seem to have associated textures, but multiple parts to color. Just go in and change these colors to more realistic values, and don't forget to use some translucency (for realism).