I think it may be a good idea to leave the French term, as in the following examples. One site describes it as a 'wild ski area'. It's more than just 'off-piste'.

At least 20% off on your ski and snowboard equipment rental ... Western like scenery, preserved nature, relaxed and homely atmosphere, ski sauvage . ...
www.winter-sports.com/

LES ARCS started out as a "SKI SAUVAGE" area. The idea was that they weren't going to have much in the way of recognised 'Pistes' rather they would mark off ...
www.goski.com/resorts/frlesarcs/resort_welcome.html?cntry_o...

There are twists and turns through rocky outcrops, and your skis occasionally glance off buried rocks, reminding you that this is 'ski sauvage', ...
www.guardian.co.uk/travel/2007/feb/11/skiing.wintersportsho...

This fearsomely beautiful and wild ski area (it cannot really be described as ... minimal lifts - and more than 7000 vertical feet of mainly ‘ski sauvage’. ...
www.ultimate-ski.com/Ski-Resorts/France/La_Grave/index.html

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Note added at 1 hr (2008-11-12 10:18:56 GMT)
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Another possibility is 'backcountry skiing'

Backcountry skiing is skiing in a sparsely inhabited rural region over ungroomed and unmarked slopes or pistes. More importantly, the land and the snow pack are not monitored, patrolled, or maintained. Fixed mechanical means of ascent such as ski lifts are typically not present.
http://www.answers.com/topic/off-piste