It worked (so far!) for me
Hi. I recently booked a pair of saver award seats in first class for NRT-IAH then IAH-IND for 110K MileagePlus points each. The NRT-IAH segment is ANA in first class (yay!). The domestic portion (IAH-IND) is United economy at this time. I found out about this award waitlisting recently and called United this morning. When I explained my request to an agent on the phone, she knew what to do, but required help from her supervisor. It took about 30 minutes on hold, but they were able to waitlist us in "IN" for the IAH-IND segment. I did not have to pay anything additional. I was not asked to pay for a change fee, though the original booking was 2 days ago. I have read here and elsewhere that not all agents know that this is even a thing, but I guess I lucked out.
Happy New Year!