In my experience, UA agents have been a lot more agreeable to wait-listing mixed-cabin itineraries - because the miles have already been deducted. The common pushback I get for usual Y->J waitlisting is the "United uses dynamic pricing and so we would have to charge you whatever today's price for a J-award is" (which is usually 250-300k) and then the fun begins where I have to tell them that they need to charge the saver miles rate and not the J-rate etc etc.