"Is it any trick other than playing with the opacity ? (doesn't seems very optimal)."
None that I know of.But it should be ok that way.
"Also I need a snow layer over them, so I'm afraid that even with 0 opacity, the snow will still appear over the hidden leaves."
Should work also,because you can assign the snow shader to the parts you want to
have snow on.
"Because of the .tgo format I suppose I can't edit the geometry inside an other 3d package right ?
Nope,not
right.You can export most tgos as obj from TG,although there are some
commercial models that have been locked and can' be exported at all.