Sounds about right for my recent experience with multi-carrier UA itineraries. IME, sometimes UA agents can get these to ticket on the spot (I don't know if they call the OA or just override the need for confirmation) so you may try calling.

Another, more creative option, could be to buy the ticket with UA only segments, wait for it to ticket, and apply your GPU. Then, within the 24hr cooling off period, call UA and ask them to change the ticket to whatever you wanted in the first place. A good agent may be able to do this 1) without a fee and 2) without losing your upgrades. I think the 24hr rule really only applies to getting a refund, but (IME) some agents will extrapolate it into a change fee waiver (for the first 24 hrs).