Wouldn't really work. Either you don't check in for the other F seats, in which case CPUs will fill them, or you do check in and scan the boarding passes (assuming you can pay for an empty seat, which is the premise here), in which case you are out a nice chunk of change. I'd guess that UA would love you for the latter case, and perhaps find a way to blacklist you for doing too many of the former case.
CPU on a transoceanic flight? (it wasn't stated as such, but it makes the scenerio more viable) Not too many routes where a CPU will move you up on thse flights and C-->F upgrades are low demand in most markets (if they weren't then y is it u r as likely to find a NRSA in F as you are in Y, but rarely ever in C?)