Well, nobody replied. Is it a stupid question...?
Nevermind, I have done some searching to reassign a key and adjusted the .pndXmodmap file. The select/ctrl key corresponds with the left control key "keycode 36 = Control_L NoSymbol Control_L".
I sacrificed F8 "keycode 74 = F8 NoSymbol F8" and changed it to "keycode 74 = Control_L NoSymbol Control_L".
After restart I tried some emulators that could be using this key (Picodrive, UAE4All) unfortunately no response.
Then I tried the F8 key in Minimenu and indeed something happened, a menu appeared.
So I guess I have reassigned the key but it only works in minimenu (bummer...)
Is there a more low level approach I could try, any other (hardware or software) suggestions?