Confirmed: GF seats are selectable at check in no matter if you're within 24 hours of the GF flights or not. You don't even need to complete the online check in, it's just that the OLCI is the only venue that allows the selection.
Maybe you should try book the earliest positioning flight, check in for it and select the GF seat, then SDC back to the positioning flight you really want to be on.
Last edited by therossinator; Mar 7, 2019 at 1:30 am
Reason: Corrected myself per post 517