Join Date: Jul 2007
Location: Homebase: CAE - Formerly, YUL
Programs: US CP, UA, HH Gold, Marriott Plat, DL, AA, CO, ++
Posts: 2,188
As for the original question...
US reservations on UA codeshares generate a UA reservation number. IF you have it, you can modify/assign seats via that.
Other than that, you should be able to find the reservation via UA's website by using name, date, etc, and then you can assign seats. Don't expect too much as far as seats though.
HTSC