Host a flashproxy-reg-http listener on s3.amazonaws.com
My suggestion is, as alternative to
Users can probable use it and won't need to find out their external IP, don't need some service to retrieve that url and don't need to mail it to someone.
Just an idea. It's up to you to decide if that idea is worth the cost/work, depending on how likely it is, that s3.amazonaws.com gets banned and on how much this improves usability.
I've also read that (some) censors don't dare to block gmail/google, but throttle it instead. My hypothesis is, that google could be interested in having their users using their services faster (so they can serve more ads). Therefore I think it's worth asking google if they could consider hosting a flashproxy-reg-url on google.com. That same goes for other pages, the censor does not dare to ban/throttle or only dares to throttle.