Thanks Dean, I really like your idea.
I've been so focused on modifying freehub bodies on other wheels to make them work with 11 speed cassettes that I hadn't really considered modifying the cassette to work with an unmodified hub. There appears to be loads of extra material on the back of the Ultegra cassette spider, so taking 1.5mm off shouldn't be a problem. I know of a shop nearby that does small jobs like that, so I'll give them a try.