You probably already figured this out but for others who may have the same question, I found that if in the WIndows Capture properties in OBS, I select Show Windows with empty name, I am then able to select 5KPlayer and get the Airplay stream. I have not tried the non-free airservers but 5kplayer (which is free) seems to work. However, since the OBS source is Wondws capture there does not appear to be any sound. The non-free alternatives (Airserver, Relfector, etc.) may be smarter about this and may appear as other than a Windows capture source. If anyone has tested, I would be interested to hear the result.