Yeah okay Bose - I think I had something in mind there.
I've had a couple of occasions where, after flying with somebody in their aeroplane - most likely just for a jolly, I've had them ask me afterwards "Genghis, you're an instructor, can we use that flight last Thursday as my biennial".
As in those cases it hadn't been briefed or debriefed as an instructional flight, and it wasn't my understanding at the time that I was flying as an instructor or as Captain, I have invariably said something along the lines of "no, because...., but I can come back and do a proper instructional flight with you next Friday if you want?". Strangely enough I've had no acceptances of that offer. It's possibly just made me a tad over sensitive about people cutting corners on biennials.
The fact that the OP is looking into the issues ahead of time, probably also means he'd do everything properly, and I'm wrong to be concerned on that point.