An interesting idea, one that got me thinking and researching ... thanks! Lots of Con(vention)s out there ... Wikipedia had lists of them.
One common thread ... expensive, complicated. Booth rental, extensive rules/paperwork to get a space, hotel rooms/travel. A hundred donations at $100 each *might* begin to cover it all. Las Vegas seems to be the epicenter, with a coast or three coming in close behind, as the location of choice for all these -Cons. CES, Comic-con, etc.
So, I backed off and thought more local ... hold these OS/2-Cons in *your* city, using whatever free resources might be available. This would get the ball rolling, and might also achieve another goal, which is to bring back OS/2-Clubs (in your city).
Again, great idea ... don't know if the national idea or the local club approach will be what takes off, but hopefully others will think more about it as well!