Actually an agent cannot do anything to prevent the system from performing a sweep.

But you can have both numbers in your PNR - the UA one for status and the AC one for mileage credit. I collect LH miles this way for years now and never got kicked out of E+.

But the problem for the OP as UA mortal is that they will not find a decent agent you could talk to and file both FF numbers in the record. So the easy bit is to book and hold with MP number and upon check in credit it to the AC account.

That must work. If the check in agent says it cannot be done, abort the check-in and move over to the next agent .