OK so to take the thought experiment further then (!!), she needs to OLCI to get a BP. You need to OLCI both tix in order to keep both F seats.

So, there's one fewer passenger on the count than people who checked in.

The extra F seat can't no-show at the gate, but scanning just the F BP's is fraudulent.

Looks like the only way around this is to get the GA involved and have them treat it (as the OP said) as if the OP was a "big person" or carrying a tuba.

Good luck with this. Certainly a tricky scenario. Report back how it plays out.