If it's a single pilot aircraft, you're either the PIC or you're not.
You need to discuss this with the owner before hand, and if you are insured to fly as PIC, and he isn't going to intervene during the flight, you can log PIC.
If he is PIC and you are just demonstrating your ability to fly, but on all the other aircraft paperwork he is PIC, there is nothing you can legally log. In this case, I would personally write the flight in the logbook, make it clear that it was only logged for interest's sake, and not include the time in any totals. I use an electronic logbook which makes this easier.
PICUS only applies if it's an exam, an integrated course, or a multi-pilot aircraft. On a single pilot aircraft it has no relevance.