Go for it mate, enjoy your trip, you are a pioneer for the rest of us and good on you for testing it out.You’re correct, the France via runs for a finite 6 mth period and allows multi entry during that time frame. My view is that if I enter and exit via a French port, that’s highly likely to be the only time anyone checks my passport. Therefore, as long as I’m within the date range of the visa, is the customs official when I leave France going to care where I’ve been during those 6 months? If they do care, do they have any easy way of checking?
And that’s a key point- are they really going to care about Brits overstaying? They may, and that will probably be politically driven. What I’m hearing and seeing, is that at present they don’t. I’ve heard of people who have already exceeded the 90/180 days entering and exiting through Malaga airport and if arriving from U.K. the customs official just stamps them in, no questions asked. It comes down to interpretation of rules and attitude to risk at the end of the day I guess.