You may want to have a look at the Nano-X framework (formerly known as Microwindows): http://www.microwindows.org/
It claims to support down to a 16-bit DOS system, so I'm not sure if it's suitable for an 8-bit, but maybe the library can be pared down to just what you need.
I haven't used it, but at one point was considering looking into using it for some simple display UI (though on a 32-bit ARM system). Unfortunately, the project shifted gears before I actually did anything with it. I'd be interested in what your take on it is (or how well it works if you decide to try to use it).