Hi Petern - off there myself in a week, and been researching the same....
Looks like the o/p in Mayrhofen itself isn’t that good, but some of the other resorts in the Ziller Valley (which I think are on covered on the Valley pass) seem to come up trumps - particularly like the sound of HochFugen http://welove2ski.com/hochfugen-off-piste-skiing
Found these quotes on other forums:
…. if there’s a powder day coming I wouldnt dream of going up Penken (main ski area in Mayrhofen). Hop on the train and go to Ramsau (Mini Japan!) or train to Kaltenbach and hop over the mountain to Hoch Fugen, which I think has the best off piste in the valley. Basically, anywhere but Penken.
Hintertux and the Tuxertal
Skiers in Tuxertal can drop into the Rastkogel and Eggalm areas….with plenty of off-piste for experts… either take the bus back to Mayrhofen or ride the Rastkogelbahn and then another six-person high-speed chair … cruise down the Hofberg/Gerent area to return to Mayrhofen. Both of these areas now connect with the main Mayrhofen ski areas.
Been living and working in the ‘hof for 4 seasons ….I can see why the locals have only race skis in their quiver, the off piste is realy inaccessable and dangerous just don’t go there...or if you do then make sure you’re fully covered with your pro off piste insurance ‘cos you’re sure to be heading home in a body bag