Ok we can now tilt 'n tumble!
I think I got everything working with the mapper and EEPROM. Motion controls and centering work, saving works, and TASing is possible.
However, I've never had a GBC or played this game, so I don't really know if the controls feel right, or if it gives the proper playing experience. So, if you are interested in this game I would really appreciate some testing and feedback. (I know there are a few graphical bugs, I'll work those out over time, I'm primarily interested in how it controls.)
Some notes on controls:
You control the angle of tilt directly with the controls. This means there are some nonlinear effects. They aren't really that noticable in casual play though, since the angles are not that large.
The control scheme is based around rotating about the up-down tilt access first, then rotating around the left-right tilt second. However, both rotations are around body fixed axes. This means that left-right tilt is coupled to the up-down tilt. The effect of this is that left-right tilt is less effective as up-down tilt increases. This might be an important consideration for a TAS, but so far in casual play I didn't see any serious impact.
The current control scheme assumes you tilt slowly (basically the only accelerations on the system are from gravity.) There are no induced accelerations from fast motions. This is one area where there might be noticable differences between real play and emulator. I can simulate them if the feedback is that it's really needed.