This is an interesting topic. I once spoke to a person in Russia who DID manage to reclaim land based on pre-revolutionary land-ownership documents. This was, however, in the Caucasus, and I do not know if there were possible differences there in terms of landownership, etc. (maybe something like crofts, for example). This was NOT a palace or serfs or anything like that!
However, it does set a sort of precedent. I know of one princess who is trying to gain a restitution in St Petersburg. I think that, of course, the state will not allow it, because it will simply open the floodgates. And so many people close to the authorities, important museums, public organisations now reside in these palaces.
But if I were the families, I would certainly try - the whole thing might eventually go to Strasbourg, and who knows what they might rule, especially as there must be a law that allows the Church to reclaim land. On the other hand, Russia is the legal successor to the Soviet Union, so it can just claim it was nationalisation - just like the nationalisation that happens in many Western countries, where people there also lose out.
