Well, there are aftermarket 3-position safeties for the Rem 700 that are similar to the Winchester M70 safety (on the bolt), so this should permit you to get a thumbhole stock. Gunsmith fitting is required, I believe, but they aren't terribly expensive.
Or, look into a Winchester M70 as the platform for the new stock. Winchester is supposed to be making these again summer 2008 (and waiting list may be quite long).
Or, get a barreled action from Montana Rifle Company and have your custom stock fitted to this action. (It's a M70 clone, available in short, long and long with a magnum length magazine)
I did the MRC option and, so far, I'm quite pleased with the result.
I think you'll like the 3-position safety. In the rear position it locks the bolt and blocks the firing pin (instead of blocking the trigger), in the middle position the bolt is unlocked but the firing pin stays blocked , and the forward position unblocks both bolt and firing pin.
One of the other claims to fame of the Winchester M70 action is the easily disassembled bolt -- firing pin assembly can be slid out the rear for cleaning in the field without tools and without fear of dropping something small. MRC kept this feature in their 1999 action.