You can try it. On other forums I've seen people try and people just won't go to it. I've even seen where it announced a highly active and old forum closing and with members with post counts over 100,000, for which the created a replacement forum everyone could go to - and it failed within a month.
There are a FEW people willing to devote some attention to the inner workings and small details of such as this Convention, but very few when it comes down to it. Lots of ideas by people, very few actual doers.
But, again, this could be done as a social group. There people can be allowed and disallowed. What seems obvious is with so few participating in this now, how much smaller would it get requiring the jump? Many folks have tried to get social groups going and few participate and not for long.
If you look at the forum, interest in topics drops very quickly. Unless there is constant new blood (new members) the Convention will go the same way. That's just how it is on every forum and every topic. People come and people go. If they can only go and no one can come except by jumping thru hoops, it dies quickly.
Here's another example. The Stephen Colbert forum (when still in existence) which was moderated was getting so massively trolled, they put in place a rule that it required approval to be a member. Just send an email to the admin who would then approve you. Not only would NO potential new members bothered, most of the long time members wouldn't either. Things aren't based upon how they ideally should be, but rather how they really work. Why is participation dropping? Because people AREN'T excluded? That makes no sense.