I thought of that but unfortunately that query only works if the photo was attached to the POW album. Since that is a manual process, and some POW photos did not get attached properly, the missing photos won't appear in the query results.
I don't think photos are flagged in any way that designates that they were selected as POW. That's one of the reasons why the POW album is valuable. The album is the only way that I know of to identify historically which photos were selected as POW.