QUESTION - Who did the entire reservation? Whoever holds the PNR for the entire reservation should be able to request your seat selection. Only at the time of check-in will UA's system allow multiple check-in facilities on the various counterpart partners' seats but then only if their system allows through check-in at the time you check-in for your UA flight.

If however, your travel agent booked all your flights for you, and even though UA is your first segment, UA may not be able to see the rest of your booking without it going into 'check-in' mode.

UALRSSRDTWRR, it really has nothing to do with how much he paid for the fare. He just wants his seats selected.