Using a PC as a GUI is not a solution because I can't tell my granny to connect her laptop to her washing machine to get some socks clean. *g* So we have to build a stand alone operatig panel without PC support from scratch on.

duh, I think your suggestion points into another possible direction -> OOP. (if you look at your structures as classes)
At the moment I check how useful this could be to solve our problem. If the display masks (our screens) are objects one could (easily?) build a system of different building blocks, each one knowing about its own data to deal with and the actions behind the assigned system events. I'm not familiar with OOP, but if I read the overviews of object oriented programming I think it may help.

To return to the interpreter language there are two points to discuss:

1. (positive):
You can describe each mask and the actions behind with a "handful" of bytes (the interpreter language commands) and you can pack these bytes sepeartely into an updateable memory.

2. (negative):
You have to build and learn the use of your selfmade interpreter language AND your iterpreter code is not easy to debug if something is going wrong (you can't see anything with an emu, depending on how complex the language is)