No, there is nothing can be done. After all, if they had gone up in value, the beneficiary would not be happy to receive only the date of death figure.
A beneficiary could, in theory, buy some sort of CFD to hedge (at a cost) some or all of the risk of a share portfolio going south between date of death and date of distribution. For a substantial share portfolio this is perhaps an option worth considering. I don't think an executor could be regarded as having a duty to undertake this sort of exercise on behalf of the estate.