Hi Marcus,
Australia to Austria will be quite a trip! But Saalfelden is pretty close to Bad Gastein. Probably a couple of hours in the car or so.
I haven't tried the Radon, as shortly after I was (officially) diagnosed with AS I left Austria for about 11 years. In the meantime (about 5 years ago) I have started on the NSD Diet which has really helped and lowered my pain level a lot.
I've come back to Austria last year and since I was busy with getting settled back in, finding a job etc, I didn't have a chance to check out the possiblity of going there. Then my husband and I started trying for a baby and now I'm pregnant, so I won't be going for a while.
I'm a member of a very active AS Self Help Group here in Vienna, and I have met a few people who go to Bad Gastein regularly. However, they are all also on some sort of medication, so it is hard to say what really helps. In addition to the sitting in the Radon tunnels, you also do a lot of exercise (or so I recall) so that probably helps too.
There is another Radon treatment center in the Czech Republic, here's the link:
http://www.laznejachymov.cz/en/Let me know if I can give any other info! All the best!
Regards, Christina