My idea was to have one global variable, a predefined value [average playcount] (something like [playing track] that you can choose as value for Album field). This could be used as second criteria, the one that could come after "Last Played not in last x days".
This way you could ban from playing those tracks that are about to be played more than average.
It wouldn't stop playing tracks with above average playcount to be EVER played in that playlist, because [average playcount] isn't constant value, and at some point in time, tracks that couldn't pass this criteria before, should pass it eventually. This way you could force all the tracks to be played similar number of times.
[average playcount] has to be autocalculated by MusicBee, at startup, or after every played track, or on user demand (if it's heavy task for cpu to be taken out that often).
This is something that shouldn't be hard to implement, and could improve auto playlists and mixer playlists.
Advanced, predefined value [playlist playcount average] could be calculated before populating playlist, and should mean the average for all the tracks that match the playlist criteria. This one, I guess, could be hard for implementation, but again, just an idea.