Found logs that keep getting added to the listing could cause someone to attempt to hunt it, in vain, much like found it logs on a missing cache would. Not many would think doing this sort of thing on a cache is right, so I would ask- what makes it so right on a benchmark?
A TB tag could be put on them, but don't release it. Keep it in hand for cachers to log as they would any other TB and explain that they can log the BM also, but as destroyed. Doing it that way still allows people to get credit in some way for it and also educates them a bit about benchmarking.