As already suggested, try to get AF to tag the bag to SFO. Chances are they won't do it but it is worth a shot. If it is tagged to SFO, you just drop it off at recheck at TBIT which is after you exit custom (turn right) but before you leave the secured area.

If AF do not want to tag the bag to SFO, you can also try your luck at recheck station at TBIT... if someone from UA is there, they may be able to check you in. However, it's not guaranteed that UA will have staff there when you arrive. In that case, you take the bag with you to T7.