Miles upgrade at Twilight check-in?

Due to fly out to Las Vegas on Saturday, and have an upgradable PE ticket, and whilst the chances are small that there will be a G seat available in UC, I thought I might try on Friday night twilight check-in and see if they'd consider it.
Does anyone know if the G upgrades that can sometimes be bagged at "normal" check-in would be available the previous evening at Twilight check-in (assuming that there was G available of course). I only live 20 mins from LGW, so thought it might be worth the trouble of packing a little earlier and chancing my arm at twilight check-in ?| ?
Likely? Thoughts?
Cheers and thanks...
Does anyone know if the G upgrades that can sometimes be bagged at "normal" check-in would be available the previous evening at Twilight check-in (assuming that there was G available of course). I only live 20 mins from LGW, so thought it might be worth the trouble of packing a little earlier and chancing my arm at twilight check-in ?| ?
Likely? Thoughts?
Cheers and thanks...