Have you seen this? www.maxi-cosi.co.uk/gb-en/Products/Car-seats/Toddler/Opal/Features.aspx It seems it can do rear facing until 18kg. They have even addressed the "where do the legs go?" argument - and looks like the belt routing is under the seat in a sort of fixed base thing. And they are selling them in mainstream shops like Boots and Mothercare :)
Is this the first step in extended rear facing becoming mainstream? :)