Hmmm... Somehow that looks like something I used in Hibreed. You intend to create a model with changeable clothes right? Me too

Now I dont know about the bmap_blit stuff , but what I did is combine the upper and lower clothes using FFP shader I adapt from multitexture terrain shader on wiki .

I used 3 layers, 1 undies, 1 upper clothes and 1 lower clothes...