Ok I got it up and running, finally found some time this week end. I will create a new fork on github with UAE4ARM-RPi that has all changes needed for Gameshell. Including instructions, example configuration files and maybe icons.
The next days I have still to try out CD32 support, find a way to switch between joystick and mouse. Or how one could change disks, like for the all new Barbarian Plus which has 6 disks.
I tested quite a lot of games, to this moment only Pacmania did show some audio and screen size issues, everything else works well.
I can now configure Gameshell keys to behave different for each single game. E.g. space key in Turrican for the energy line on Y button. Or in Hybris have Y for ship expansion, and A for smart bombs and B for pause.
I guess Gameshell has not enough keys for FA-18 Interceptor, however I have not yet given up on that one
Some parts are coded and not flexible right now, so I still need some time to make it configurable by using the existing UAE configuration files.
Finally, although UAE provides an in game menu, usually opening with F12 however also configurable, it crashes once I press it. But this one is not that important for me yet. Priority is to be able to play games.