... Forgotten Worlds.  ...
I don't see a reason to ever not be firing in that game. 
To move your pod in front / in back.  For blocking and getting around/through walls/floors/obstacles.  That blocking 
really helps.
The game internally already supports auto-fire (that is holding down the button generates a continuous stream of bullets) so an auto-repeating circuit wouldn't be needed.
Right, so you don't want "auto-fire".  You want "continuous button press".  "Auto-fire" is something different, and there are strong feelings on it; readers might get confused and give you views on the unrelated auto-fire instead of what you want.
Is there a way to get Mame to do the auto firing [continuous press]?
AFAIK, not without editing the code.  For FW and what you want, I'd change the IPT_BUTTON1 to IP_ACTIVE_HIGH (from IP_ACTIVE_LOW, line 674 in mame 0.110u2) and recompile.  This would make it so you would fire 
unless you press the button, without effecting other games.  It's a hack, though, and will not be accepted in official mame.  Just like auto-fire.