Update on this. It looks like the building (its name is Buzludzha) is set for a restoration. There's a team of European restorers there right now reinforcing the remaining mosaics and covering up the many, many holes in the roof to safeguard them against further decay. It honestly could not have come at a better time, as it's estimated that one third of the mosaics have already been lost. I would show some examples but I think hotlinking is still disabled on this forum so just google "Buzludzha mosaics".
However, it's apparently not a full restoration, merely just saving what still exists.