It turns out that if we do not enable RSVP, then the event is visible without login.
This may remove the main obstacle.
If necessary, we can use a dedicated form to mark participation, as @zackteo does in Clojure Asia events.
(Example)
Even if we use this practice to avoid the invisibility problem, we still have the problem of the wrong time zone being displayed (last Saturday, this made a few people show up one hour earlier to the visual-tools meeting).