I booked a UA award ticket to travel on LOT and couldn't do seat selection. The last leg is on (India based) Vistara airlines. I was able to do LOT J seat selection using their Manage Booking. The UA award ticket had the Vistara record locator also listed. I called LOT to confirm that seat indeed took.
https://www.airvistara.com/in/en
Not sure if this is an option for seat selection for others. Don't know if it will take if you aren't flying a segment on Vistara.