Sorry, Anon, no dice for the time being. I thought there was a permission I could give you that would override the board limit, or maybe give MB an exception, but the controls provided in phpBB aren't that fine grained.
There are modifications I could make to the board software, but I don't have the time now and won't for at least a month -- I'm moving out of my apartment in 4 days, and I'll be on the road all summer. I'm happy to make any edits you need from now until June, at which point my contact will be sporadic and you'd be better off asking another mod.
I know the current editing policies aren't ideal, I have it on my post-move list to make the necessary modifications, but that will be mid July at the earliest.
User Control Panel > Board preference > Edit display options > Display signatures: No.