Waited for a few days to see if there are any brighter minds out there that can answer this question definitively, but since it seems nobody has figured the answer yet, I will hazard a guess and say probably no. I’ve tried it in GT5 after finishing one of the 24hr endurance races in campaign mode, only to discover the button wasn’t there (IIRC), or the button was disabled. I think GT7 will probably be the same, given the massive file size, but it’s also probable that PD may have tweaked something and made it available.
Custom races may be a bit different from the Mission races in GT7, but I still don’t think they would make it work.
I could be wrong though, I’m just saying these out of experience.