Turbo Quattro

Yes on firmware <= 200 you can run an encoded script (if green menu is enabled). This is the replacement for firmware >200 and AFAIK a downgrade is not possible...

*bump* Anyone with ideas? Have useful information to trade if that helps, for instance how the public keys are created and can possibly replaced by own private/public keys

In Discover Pro firmware 388 it is possible to execute a script named autorun placed on USB stick:
from etc/mcd.conf:
[AUTORUN]
Callout = FNAME_MATCH
Argument = /autorun
Match Rule = INSERTED
Fail Rule = SW_UPDATE

However autorun is only executed when there is a matching signature in file called ...