Your chances are definitely better on a Saturday. I'd chance the SFO-ORD option but I would not on a weekday though.

I just flew the DEN-ORD 3 class 767 last week in C.. I changed my routing from the confirmed SEA-SFO-ORD to standby SEA-DEN-ORD because the SFO-ORD was full in C. But it was a Tuesday. My upgrade DEN-ORD cleared at the gate.

The meal in C for my lunch time flight (10:47am departure) was a tiny fruit and cheese plate. Not even any meat.