wot sort of MTB do you want??
DH, FR or XC....
you need to look around the www for tignes, cham, les gets, morzine, morgon, chatel, verbier for mtb then get advice on here for allfroid, magic wood and other areas i have no idea on.
there are LOADs more tracks in the alps, gravity-slaves.co.uk have a series of articles covering road trips last year...
i have heard allfroid(sp) is only 3 hrs from morzine (the only area i know well for MTB - really bloody well -) in the alps.
i thought about a similar trip this year, but my mind is now set on 'extreme XC" sort of stuff this summer, snowdon, skiddow, kinder and jacobs ladder, helvellyn and mybe the ben itself - plus some new trails near wrexham look promising.... OK, not all bridleways, and can be done on regular MTBs; but on FR bikes with armour you can go f8ckin mental on some of these runs!! AND,it's all nearish good bouldering so that I can take the odd weekend off, not just the one week for MTB... thus get more climbing done whilst riding my tits off on a FR bike!!
hope that helps mate.