Unfortunately I have been unable to trial the solution that I proposed as my customer went ahead and had his paths relaid with fresh gravel however I now know that this stuff can be tracked in on the feet of visitors as I now have it on another customer's site and I think I was the unsuspecting carrier. It does disappear in dry weather so inhibiting light and moisture will help to eradicate the problem but this may not be possible over large areas or where there is a mixture of the algae and other plants growing together.
In the instance I originally quoted, no weedkiller of any sort had been applied for several years as we had been harvesting lavender seedlings from the path for at least two seasons.
So the upshot is - no positive news at present . . .