One solution we might use to get people to migrate to 3.6.10 would be for two people to each start a game, one that's the real game running 3.6.10, and another that's left up for 5 minutes before each mission running 3.6.9 solely for the purpose of telling people who join to grab 3.6.10 and join the real game. It would be a bit of a bother, certainly, but we'd only really need to do it for two weeks or so before the regulars are all using the new version. And if it means the quashing of bugs that might otherwise make it into 3.6.10, it'll be worth it in the end - Hell, I'll even volunteer to run the 'Help Desk' game.
Also, yeah, having the website display 3.6.10 instead of 3.6.9 would help too, since people often use that to see if they should bother logging on or not. Showing 0 people online when there's actually 7 people playing wouldn't help adoption.