I think as long as they're not horribly macabre or graphic it should be OK. Most historical postmortem photographs are very old, black and white, and therefore quite blurry anyway. For example, there are several pictures which are not at all graphic, and which I wouldn't have a problem using on one of my own hubs, of the men who were shot at the O.K. Corral. More recently, I wouldn't have a problem using a picture of Pope John Paul II lying in state because everything was done very respectfully. Just use your best judgement and I think it should be alright.
If I am wrong and there is some specific rule against this, I hope someone will correct me. But then again, that sort of answers your question right there: I haven't come across anything that says NOT to use these photos either.