For now, I'm just going to do this:

• If a project has no hashtags, don't offer to autocomplete it.
• If a project has no primary hashtag but has at least one other hashtag, autocomplete with an alternate hashtag (chosen by some chaotic internal process).

In the future, we could give milestones implicit hashtags like #pluto/3 (to mean "The milestone with internal ID 3, in project #pluto"), but I'd like to see more interest in autocompleting milestones first. I think the big issue now is just that the thing pops up but doesn't work, which is confusing.