As a side note, no, you cannot built a graphics card driver from the UAE C component. That doesn't depend on the SDK. It only depends on the 68K interface part that, more or less, just traps into UAE.
It would be easy to make the traps not go into UAE but whereever you like. You could patch LoadSeg("uaegfx.card") to replace the 0xF0FF60 jumps. So you probably could write a gfx card driver without needing to know ".card" internals, but only things from UAE.