I've done this on many occasions. However, you must ensure at the gate (or at the RCC) that the FF program you want the segment to be credited to is in the proper field on your booking. UA uses two fields for FF numbers, so it can accommodate the UA/MP number for flight benefits and status, the second is for the program the mileage is to be credited to. Always request a new boarding pass to confirm the non-MP number has taken to the segment. But you must confirm this for each segment, immediately prior to that flight. (You can even hold an elite upgrade using e500s or CR1s or SWUs while having the flight credited to a different STAR program.)

Also, if the flight happens to get credited to UA and not the program you want, call MP immediately and request they remove the flight from your account. As long as your boarding pass shows the other STAR program code, then you can mail the stub and ticket copy to that program and request the mileage. However, you must monitor your MP account within a day or two of the flight to catch a mis-post. Any longer than a day from the time the flight posted, MP will not remove it from your account.