After further inquiries, I'm going to answer my own question for the benefit of anyone else.

I was able to assign a seat on this Royal Air Maroc flight (Qatar codeshare) only through the Qatar call center. Not through RAM, and not on the QR website or app.

Perhaps the giveaway is that with 30 business class seats in the cabin, as indicated by Aerolopa, only 10 show up on Expert Flyer under the RAM flight number. It's as if the others don't exist. One of those missing seats was assigned by Qatar...BUT...BUT...only after the Qatar rep said I'd have to call RAM, and I pushed back saying RAM couldn't see my reservation even at their call center. I asked QR to send me an e-mail confirmation, and sure enough it shows the seat number.

Now, I went back to Expert Flyer and entered the Qatar Airways codeshare number for the flight. Sure enough, the 20 missing seats showed up here. Normally, you wouldn't be able to get a result with a codeshare flight number.

Advice for Qatar Airways: in such a circumstance, allow for seat selection in your app and on your website. Don't make customers dial your call center, which almost certainly is more costly.