BrazilBusDriver

My reading of 14 CFR 1.1 would agree with yours:

*Second in command means a pilot who is designated to be second in command of an aircraft during flight time.

*Flight time means:

(1) Pilot time that commences when an aircraft moves under its own power for the purpose of flight and ends when the aircraft comes to rest after landing;

Pilot time isn't defined in 14 CFR 1.1, but I'd bet FAA legal would argue it doesn't need to be because it's obvious. I'd be pretty wary of putting anything other seat time in my logbook unless there's some legal interpretation that says time spent in the supernumerary seats counts as "pilot time".

Edit: formatting/words
Edit 2: Well actually Part 61 does define pilot time:

Pilot time means that time in which a person -

(i) Serves as a required pilot flight crewmember;

I think the operative word is "pilot" here. Hard to argue you're piloting anything from the bunk.