Hi Juergen,
In the documentation section about possible problems with ATR files, the issue with modified OS/SIO is mentioned along with a few others.
http://www.atarimax.com/flashcart/docum ... pter7.html
Because there is no /RESET signal to the cartridge port it is not practical to have a software reset, other than a power-on reset for 8-bit cartridges.
You could add your own reset switch by interrupting the power to the cartridge port or the power trace on the cartridge, which will force the cartridge back to the default bank. Pressing reset after that may return you to the menu, if the OS stopped due to the cartridge interlock code.
It is possible to add the code you mentioned, but it might decrease compatibility with some games. There are also only a few bytes of space left in the EXE loader code, which might be a problem.
Steve