I've never been to a convention, but it sounds neat.

My impression, from experience trying to organize stuff, is that the best way to do this sort of thing is to first poll people close to you to see if they think its a good idea, then if the stars seem to be aligned, just go for it and tell a bunch of people until you start getting a critical mass. When you get people you say yes privately, then ask them to say yes publicly, because it might not occur to them otherwise.

On the basis of looking at past meetup posts, I figured this attempt would count as a success if I got five to ten "non-Jennifer comments", and so far we're at eleven!

One thing that's helpful is to model the plans in terms of other people's interests instead of your own, and imagine people who might not know what's going on and explain how the thing you're proposing would connect with the simpler and more public things that they are probably already interested in. For example, I don't even know what TAM is, but googling turned up conventions for the Telephone Association of Maine. I assume from the name that DragonCon is something to do with Fantasy, and maybe its in SoCal, so I didn't bother to google it.

A good way to do this, then, would be to do the research on your own, figure out upcoming dates and locations, think about logisitical barriers and other people's interests and then say something like:

"Convention W is happening at X on date Y with amazing thing Z. On the other hand convention A is happening at B on C with amazing thing D. I want to negotiate a group rate for hotel rooms for LW people that are near each other [or some other thing you can do to "pitch in and help" to get the stone soup started] , but I want to do it at the one that will have higher attendance. Which of these would people prefer?"

This would give people information necessary to respond, they could come up with improving variations on your plans, and so on. Then, look for ways to say yes.