Would it be feasible to add "any" 12v (ambulance or police style) beacon bar? Where would one need to tap into to get it integrated with the game?

How does the "official" topper connect?

Here's what I have done. I found some NOS BetaBrite LED signs. Apparently these were used in Stern Spike system for Tournament modes. Unfortunately this is not supported anymore, but the screws in the top of the backbox are still there and they fit. So I manually programmed it with the texts that can be seen on the side of ECTO-1 in Ghostbusters 2:

GHOSTBUSTERS
FOR HIRE
CALL JL5-2020 (not the same number as in part 1
WE'RE BACK
WE'RE READY TO BELIEVE YOU

Nice custom topper for under $50.

I have received a number of PM's from people who want to attempt the same.

You need to look for a Betabrite 1036, that's the exact make and model that Stern used for the Tournament Topper, e.g. ebay.com link: BETABRITE LED Message Display Sign 3 13 16 H 10361211 New Open Box Nice

Make sure the mounting brackets are included. If not, you need to fabricate something yourself that still makes use of the screw holes on the top of the backbox.