Actually, you can make SOME textures invisible by coloring their alpha layer pitch black in photoshop (open the texture, go "channels" select alpha layer, brush the whole thing black) However this only works for textures that already have an alpha layer set (you can create one for any texture, but it won't do anything, other than setting the whole thing white)
Some of the textures that I noticed that have an alpha layer: Dante's/Vergil's hair (as in my avatar) Kat's clothes/armband thingy. And, well, those are the ones i noticed, didn't really play around with much besides the main characters clothes. But you can easily determine if the texture has an alpha layer, when you are in logging mode in texmod and you browse the textures, when you select one that has an alpha layer, it will become invisible in-game, that's how i found out anyway.
Hope this helps