Saturday SFO-LGA service.
UA has some SFO feed. If it works, then LAX-LGA could be tried.

How about SFO-SLN-LGA, SLN being Salina, Kansas which has United Express service and a very long runway. Price it so there would be no SLN passengers and the plane either touches and go in Salina or overflies it. Alternatively, it could be priced so SLN passengers board to go to LGA but no SFO passengers leave, thus not having a security risk. On the return, price it so there are no LGA-SLN passengers but only SLN passengers boarding to SFO. LGA-SLN passengers could fly a LGA-ORD/ORD-SLN route.