I cannot see what is illegal about a syndicate member allowing somebody else to take the controls and log it accordingly.
I thought the poster said that only the syndicate member was covered by the insurance, but actually he only said
For the purposes of insurance, administration etc, the group member is the PIC.
But insurance is the key to this. You can't fly an airplane if you don't know if you're insured or not, in the same way it's foolish to drive a car without knowing if you're insurred or not.
Many insurance policies only cover members of the group/club without actually naming them. That's the way or policy is worded. Only our club members are insured + anyone flying with the explicit permission of the CFI.
If he's not insurred then it's illegal.
One problem would be if the syndicate's insurance is for specified (named) pilots only, in which case this flight would be illegal - because insurance would be void and insurance is mandatory in the EU. But, if the logbook entry is made only after the flight has successfully landed, it's hard to see how this could be detected.
Well, that's like saying you can drive my car but are not insured, and so long as you don't get caught, then it's ok. Illegal is illegal. If these flights are going to make up "reasonable chunk to my total time" then best that they aren't illegal flights, even if he didn't get caught.
Best to check out the insurance situation before doing this. It should be easy to do, as the insurance documents are probably kept in the aircraft.
dp