I guess the edge case where this happens now would be domestic connecting C+ legs on intl PE fares, right? These book into W domestically but if I’m thinking about this right, P/A/G should be ahead of MC fare classes in priority (within status level).
It should still have an underlying main cabin fare. Just like when book F and connecting flight is CRJ. You're booked into a Y fare class, despite paying for a through F fare.