Using the people of the Soviet Union as allies is often discussed in alt-Barbarossa scenarios. I don't know that using them as a huge manpower pool is really their best use. Instead promising freedom from Bolshevism would, ideally for the Reich, result in elements of the Red Army dissolving. I think I read one time that Soviet units were purposely kept diverse in order to prevent this, but I could be totally wrong. In addition, such promises might result in sabotage by people still in Soviet territory who want to see a change in governance. Military factories might be damaged, or stuff like that. Maybe it's unrealistic, but I think it's probably what the German military would be looking for rather than a recruiting a bunch of disaffected minorities.
As for the promises being valid, I think the Germans could make selective promises. The Baltic states are fairly small, so promising them anything is largely pointless. But the Ukrainians, Caucasian people, and maybe the Belorussians, are all good targets for convincing, whether the promises of freedom are sincere or not. Oh, one other thing. If some of these groups are friendly to the Germans it might reduce the amount of garrisoning the Germans have to do, which might help in some way.