Even if you have a full type rating in the plane that you are flying, if you are not the Captain and the one that signed for the airplane you NEVER log pilot in command time. IF the Captain is not able to continue his duties as PIC (such as he dies) then you are the only one left and have assumed command and can log that time as PIC but id recommend putting a note in the comments.
As a first officer you log total time and SIC time for the entire duration of the flight regardless of who was on the controls.
What are you basing this on? I have copied the FAR which is very clear. I have been hired by seven airlines over the years, 5 of them majors and this has never been brought up. The FAA is the governing authority, not a possible future employer. When you complete an application, they will have you place your time where they want it which is fine, but this is how it should be logged. Please site your reference.