Individual near clip planes for cameras

I’m trying to make a portal system similar to that of the Valve game, and I’ve got everything working except for the clipping of objects between the scene capture camera and the portal. What I really want to do is oblique near plane clipping, but it looks like getting to mess around with a camera’s projection matrix will need me to build the engine from source, which I’d rather not do. At the moment, I’m literally just iterating through all the objects in a scene, and hiding them from the scene capture component if they’re on the camera’s side of the portal, but this is very clunky and breaks in some circumstances. Is there any way to edit a camera’s near clip plane without changing it for the entire project?

I have the exact same problem