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?