That could actually support my tentative suggestion - Isabel -> Isabeau, so why not Ma(g)del -> Madeau? Interesting!
And, even stranger, it seems to happen in South Africa as well. Perhaps you are aware that we've got a very odd Minister of Health who has made a career of encouraging HIV-positive people to eat lots of lemons, garlic and beetroots to build up their immune systems, and has not quite got around to providing the kinds of drugs that actually work ... she's a national embarrassment and a cartoonist's dream come true, and her name is Manto! Surname is Shabalala-Msimang, which just indicates that she comes from one language group and her husband from another, like Papadimitropoulos-McGregor. No linguistic clues there. A lot of black (South) Africans have Western given names that sound old-fashioned, because they go back to the days of colonialism and missionaries, but I've never encountered a reference to Greek Orthodox missionaries out here! (Except my friend Dimitra, but she's a free-lance ...)
Thank you for widening my mental horizons - what fun!