You could use texture lookups (with GL_NEAREST filtered textures) in fragment programs and implement your “own” filtering method. What effect do you want to achieve?
I’d like to implement my own texture filter “into” opengl without writing a software renderer. I only tought of a project to learn from, it was only an idea.
i am also looking for a way to do this, but one that would work on nv2x and higher cards.
an example was posted in another thread here: http://www.opengl.org/discussion_boards/ubb/Forum3/HTML/011143.html
that might allow you to do it in FX class hardware, but so far i have seen no advice as to how one might tackle it in pre FX hardware (it’s probably not possible).