The work-around I have found for this is to enter the nether in creative mode (if you're on a server, have an admin do it). I then reloaded the chunks and went back into survival. Unlike despawning (which doesn't seem to change the anger status) or entering peaceful mode (also just despawning the mobs), creative seems to force the anger to be reset (mobs present but non-aggressive). I have not had aggro issues since, nor have the other players on the server. Although this would obviously have to be repeated any time someone hits a zombie pigman, it seems to be a pretty solid repair for the specific instance of the issue when it happens.
The work-around I have found for this is to enter the nether in creative mode (if you're on a server, have an admin do it). I then reloaded the chunks and went back into survival. Unlike despawning (which doesn't seem to change the anger status) or entering peaceful mode (also just despawning the mobs), creative seems to force the anger to be reset (mobs present but non-aggressive). I have not had aggro issues since, nor have the other players on the server. Although this would obviously have to be repeated any time someone hits a zombie pigman, it seems to be a pretty solid repair for the specific instance of the issue when it happens.