I am so confused foxfire.
I didt mean geometry modification via complex vertex shaders. I mean if there is way not to really do it but make it appear like that using a view-dependent method. i have no exact idea how should it be. but i think its possible.
let me give you a deeper plan:
we have two objects (models) one to be subtracted(B) and one as knife(B)
firs we render (A) to stencil buffer then using stencil buffer we render (B) on screen with flip normals, so til now we have rendered B with fliped normals only on pixels were A and B are overlaping (interrelate). Then we render Last B once again using stencil buffer but instead of screen we use stencil buffer as render target. finally we render A using stencil buffer on screen. after all we have a rendered total screen. A and B are:
1-B is not rendered but in interrelate areas it is rendered whit fliped normals
2-A is rendered but in interrelate areas it is not rendered

it may have some problems specially with texture and UVs.
I think it may work but i dont know how to implement it. i hope you understand my approach.