I have a simple GUI I wrote using Codewarrior for HCS12(X) and also use it on Coldfire platform. It only supports small monochrome 128x64 LCD now. It can run as main thread or use RTOS Idle Thread. Using a RTOS allows the GUI to only consume processor time when the idle task is running this keeps critical time tasks predicatable and the user never knows the GUI interface is being stalled. Provided the system is not overloaded and has free time.
Link to a few pics of one product using the GUI code, scroll down page for a few screen shots
I am not sure how well it would run on a 8-bit processor.
Do you have a basic driver written to communicate with the LCD? I only have drivers for the various LCD's I use.
I would help you out with some basic code to draw lines, circles, rectangles, simple fonts and sample driver for 128x64 LCD.
The entire GUI is approx 28,000 lines of code. The Codewarrior compiler and optimizing linker keep the code size in check though.