You may choose to log your legs as an FO as PIC, but I suggest keeping a separate column because you better be ready to deduct all that time when applying for a job at a major. When applying for a job, they only want to know of your PIC time acting as a captain (not part 61). So if you are logging PIC as an FO, get ready to remove all those times.

A great example is to just take a look at the FedEx application page - they clearly state that they are only interested in your PIC where you were the acting Captain (to meet the PIC requirement).

You can log PIC as an FO, but you are not the true PIC for the flight.