So far as I am aware, the requirement to have retro-reflective (can reflect in all directions no matter where source of light) stickers is specific to France only, and is a real regulation. The only helmet manufacturer who provides these stickers as standard is Shark (a French brand). The French Police did fine some motorcyclists in and around the Paris area last year, but I think have otherwise largely ignored it. The regulation actually states that the stickers must be affixed so securely that removing them will damage the fabric of the helmet. Now what policeman is going to deliberately try and damage a helmet on the side of the road just to test this, thus rendering the helmet useless? Thanks to the w@nkers in Brussels for this one!!!
You could get retro-reflective stickers in black from Bike Quip (in black or red - see Rear Reflective Kits at
http://www.st1300-accessories.co.uk/), which you can then trim to size and stick on to helmet which (provided is a good quality helmet) will not compromise the shell.
Alternatively, avoid the Paris area....