<font face="Verdana, Arial, Helvetica, sans-serif" size="2">I've got a ticket from SEA to LHR on a "direct flight" that stops and changes planes in ORD. I have different seat assignments on the SEA-ORD and ORD-LHR segments, which should produce two boarding passes.
Within UA's system, will this break the direct flight into segments and thus provide full mileage credit?</font>
Jeeves, in my experience you will only get mileage credit for the one flight number's point to point distance, and not as a split itinerary.
You will indeed get two boarding passes, but on the second one you will see the wording "Re-Boarding Pass" printed on it.
Unfortunately that's the mark of doom for this type of attempt, and does not make it a true multi-segment for purposes of more miles, in my experience.