You can log anything you want, but the question is will the FAA and/or an airline or other potential employer give you credit for that time...
In my opinion: The safe bet is that total time must be either SIC or PIC. SIC is ONLY logable in an aircraft that is actually certified for two pilots. If it is certified for single or dual, you can always elect to fly dual pilots, enabling the SIC.
I don't know of any any airlines that give total time credit for FE or military backseaters either.
Most employers would be annoyed when they audit your logbook and find out that some of your claimed "total time" was really just an airplane ride.