I've been searching for a while and found some people suggesting to use iDirect3DDevice9::SetClipPlane, but I'm not 100% sure how to use that or if it would be the best thing to use. The important thing to note is that the scene is not 3D at all, this is for a completely flat 2D application.
EDIT: Feel free to close this thread. Spent the whole day getting clipping planes to work, which works fine until I realized that clipping needs to use weird funky shapes so it got a LOT more complicated really fast. I did, however, find a free third-party library that handles clipping all sorts of things that I'm going to try to use when I have free time in the future. Library is called Clipper.
Edited by Shenjoku, 30 August 2012 - 07:02 PM.