Such a post wouldn't be appropriate on the main site. We have had questions about meetups in the past and all have been closed and deleted. This has been discussed before...
Clubs/meetups: in scope?
and the consensus is that they are off-topic.
You are right that we don't have community ads, because we are still a beta site. You could try putting community ads on other sites, but they may or may not be accepted.
Mentioning meetups in the chat room is definitely acceptable. As you say, our chat room isn't very active but we do get a fair number of visitors to it, even if they don't say very much. I would try it there.
As to whether you could post on meta, I would be inclined to say yes. Other sites on StackExchange use their metas to post announcements about meetings or competitions, notably Information Security, so we could too. As the mantra goes, "meta is different", you may be successful.