An alternative to the blog trick would be something like geocaching:

(see also )

I would suggest some kind of mystery/riddle setup, with simple clues initially that turn into more complicated ones over time. Since Eo is fairly gamelike anyway, it would be a good fit, and at-sight ability isn't necessary. Geocaching only requires a few dedicated and widely distributed people to make it work, so most of the established auxlangs could manage it.

For that matter, a kind of Web-based treasure hunt where you have to find a specific page based on clues would also work.