Just interested to know how it works out in practice.
From what I have seen its a hodge podge of methods most of which are technically illegal. The person being checked always ends up paying the full cost.
You have option
a) owner PIC fills tech log out as such doesn't pay anything
b) PIC signature left empty until after the flight.
c) Checkie PIC fills tech log out and technically isn't insured becuase they are not checked out yet.
d) Checkie is outside 90days but still signs as PIC and owner goes along anyway.
As a FI you stay as far away as possible when one these flights is going on. Don't offer an opinion as its none of your business. Some airfields it is very common practise others the School CFI see's it as someone stealing his revenue and its a cloak and dagger operation.
Its why there is an ever increasing number of groups with members who are CRI's. Its a nice enjoyable thought provoking course for experienced PPL's, easy to keep current and keeps everything easy legal.