The way to go is to form a committee of dedicated like-minded people from both forums with the drive and time to make a Pinball Expo happen. The exact legal structure would be best left to more instructed minds but I imagine an incorporated not-for-profit organisation could be the way to go.

The committee needs to be entrepreneurial enough and inspiring enough to find the money to underwrite the event, build a website, organise people to bring machines, organise techs for the event, organise raffles and so on.

Start small and build. Don't need a lot of dollars for advertising for the first one, just get people from both forums to attend and get them to advertise to their friends and a bit of paid web advertising could help. Send press releases to various media, maybe you will get a few bites. If you can get some TV coverage at the event all the better for next time. Maybe someone knows people in the industry?

Sorry AJ - not being a smart ar$e but that's GOT to be a gee up - right?
Nearly 100% of what you suggested has been attempted with APA , nearly word for word 'business plan' even down to having the legal side of things covered and a website, being a non profit organisation etc ......and it is just floundering as a committee

I apologise if it ISNT a gee up and you were unaware of APA but mate - that aint ever gunna happen in the NEAR future IMO 
Hence the opening / contributions to discussions for 'other ideas'