Hi Jules,
I emailed with the host there. Camping is OK, no cost for that if you eat/drink something in the inn.
Most people will sleep indoors. Uwe and I are camping.
I dont know about the number of participants - the Gamsstein rally in Tirol (where Hahnbaumalm is the replacement for) had usually 40-50 vehicles and 3-5 tents.
Campers can use the inns facilities which I expect to be basic (they offer overnight stay in dorms, so there will be toilets and if we are lucky there is a shower for the girls

)
This is the location, you may googletranslate it:
http://www.hahnbaumalm.at/It is 1117 m above sealevel.
There is a 3,5 km sledging track.
Andy