Guys,
First of all I understand the differences between FS9 & FSX and of course, for those whom FSX is their first or only sim, some of the compatibility issues do make a difference. Certainly a gathering being FSX only has it's advantages and may even get converts.
However, not every wheel needs to be reinvented.
Currently there is a long-standing group (primarily FS9) that has done this sort of flying for ages and are on the SOH server most nights. "Flight 19" always creates a theme, then flys as many ongoing legs as required to flesh out the story line. Compatibility is more a matter of having fun than rules and it's an ongoing history lesson as well ( besides, you might convert some to TeamSpeak

). There will always be 'invisible aircraft' until everyone owns every plane but the fun is being together.
As for the warbird style 'event' take a look at the beginning of the 2010 RTWR -
http://www.fsrtwrace.com/rules.htm
to see how it fits into something larger. We had 8 'pairs' airborne in the 2 1/2 hour period. This had "rules" because it was to accomplish a goal for the race, but was also a companionship-builder. The post-race celebration had over 20 aircraft from 4 forums crossing the channel
http://www.sim-outhouse.com/sohforums/showthread.php?t=32202&p=364915&viewfull=1#post364915
(and a few German stragglers popping up when they could muster gas and pilots

)
While not intended as a mass, online group flight the recurring RTWR-style fun events hosted by SOH often turn into multiplayer travels (2008 Nobel Race around S. America, 2009 "Evita" Race to Argentina). Learning new planes, realistic flight ops (as opposed to balls-to-the-wall engine busting), navigation, geography, history all blend together in these. Even after the rebuild of the forums there are a lot of posts in the
Around The World & Multiplayer forum from these and other gatherings. Some have 'rules' some are just an excuse to get together.
Think about taking part in the upcoming SOH-hosted
"The Pacific Rim Nifty 50s Jet Challenge" http://www.sim-outhouse.com/sohforums/showthread.php?t=34599&p=391724&viewfull=1#post391724
Two things I'll suggest... slip out of the FSX forum and explore the rest of the Outhouse, even just for inspiration, or to find something to participate in; get a couple of like-minded evil geniuses to suggest a theme for a one-time event:
"Hmmm.. let's have a "Gathering of Mustangs" and ferry them from the factory to xxxx" -- or maybe the ATS Class 44A graduation flight from YYYY to ZZZZ in Texans ...
once started I think you'll find it just grows - as long as you keep the public informed. As time goes on it could be an ongoing flight/mission over several days or weeks with the ability to pop-in/drop-out.
Rob