I know of no situation where that would be the case.

When we have an entitlement to a FC seat the entitlement only applies at the time the deadhead is booked (usually by the 18th of the previous month). If the seat isn't open at the time of booking then we don't get it and we go onto the upgrade list BEHIND all revenue upgrades which means that the chance of clearing is very low.

Something else, in these situations, is make sure that the pilot in uniform really is a United pilot. At my previous airline I was home-based so that airline bought revenue tickets to position me for my trips and I frequently travelled in uniform (I was PP on UA and PM on DL). There's a lot of that in the corporate world, too, with Netjets and similar operators. These pilots fly as paying passengers weekly, quickly build status and are entitled to upgrades just like any other paying passenger. Many passenger see the uniform and assume that the pilot works for the airline operating the flight. That isn't always the case.