Show up early and ask to be put on the first class standby list (verify with them that this means by default you will also be on the coach standby list).

Then it's essentially the same as a BF UG scenario, except you don't have a confirmed seat. If there's a seat in coach, they'll clear you into that first. You should still remain on the BF UG list at that point, but only for F. You might want to converse with the TA/GA to make sure your understanding is correct. And as always, you can just tell them what your goal is (first, to make the flight, but hopefully to be on the upgrade list as a Plat as well).

I used to do this a lot coming out of Chicago when there were nearly hourly flights. I'd book something in the evening for a return, but if my business was done early, I'd try and slide onto a mid-afternoon flight.