You can break the fare on this and get two segments and actual miles. Have your TA book each segment individually. I don't have any experience getting UA to do this, but TAs will do it with no problem
Yes, I have done this, but was answering the OP's specific question. I've also found using UA's multi-city feature, I've been able to get the bomb to cough up what I want, though it's a crap-shoot.
One route I follow which has this nuance is LAX-LHR...
The non-stop is:
Flight
Airline: United
Flight: 948
Date: Fri, Jun 9
Status: NOT DEPARTED
Aircraft type: Boeing 767-300
Miles earned: 5456
The direct flight through SFO is:
Flight
Airline: United
Flight: 930
Date: Fri, Jun 9
Status: IN FLIGHT
Aircraft type: Aircraft change
Miles earned: 500
Flight
Airline: United
Flight: 930
Date: Fri, Jun 9
Status: NOT DEPARTED
Aircraft type: Aircraft change
Miles earned: 5368
One would appear to "lose" 412 miles and one segment here.
One the positive side, IIRC, one flies 744 metal out of SFO, rather than the 763 metal in the above example (though the other LAX-LHR non-stop is 772 metal). If given this choice, I'd likely attempt to break the fare and try for the 744.
Pat