I'm also interested in the correct answer to this question. I just end up sticking a triangular patch of insulating tape across the middle of the headlights with the thick end to the nearside. I don't seem to upset continental drivers, but the light available is significantly reduced.
BX14TE St Tropez 1990 - now sold
Xsara Forte 1.4i 2000
Kawasaki GPz550A4 1987
FWIW, when I was driving across France, Italy and Sicily en route to Malta last year, I didn't put anything on the XM's headlights, which have had a HID conversion. No-one flashed, pulled me over or anything else. Stick-on things were readily available in the shop on the ferry however.
HTH
Ray The Fleet (most recent first):
2000 Citroën XM 3.0 24V V6 Exclusive Auto (pre-MOT)
1997 Citroën XM 2.0 TCT Exclusive Auto (for sale)
1979 Citroën CX 2.4 EI Cmatic Prestige (slowly being restored)
1992 Alfa Romeo 164 Lusso 3.0 v6 12v Manual (on the to-do list)
It is best to show willing. If it's visible to the Gendarmes at least they can't pull you for it.
From Citroen:
This is an A4 Sheet, so print it off full size and your template should fit. Use black or dark grey fablon, or match the coverage with black tape.
Last time I had a stick on set which had a huge list of instructions for different cars, every single production model ever it seemed, apart from the BX of course. I just whacked em on where they looked right.