@paul_uk I used to have a spare button assigned to TAB, so it is indeed possible.
From memory, you should have your spare button assigned to something in Retroarch (I believe this can be done from Emulationstation) that won't be used by a lot of games: mine was R3, for example.
Then configure Mame to show the menu upon pressing R3 instead of TAB (which can be done in that very same menu TAB brings up) and your spare button is mapped.
EDIT: just to note, this wasn't with controlblock. All my buttons are connected to an RPi with a Dragonrise usb-encoder, so I don't know if your setup will work the same...