Wouldn't it be easier to just machine a cassette and freehub combined and sav even more weight off it? Obviously changing cassette would be a bit more tricky.
erm, as Njee points out, that is what the cassette project is all about. originally it was just an engineering project to see what could be done. the proto was all alu and scheduled for release on the original dates but wasn't durable enough during testing, likewise the 9t made the chain run too rough.
The new version is Ti and steel sprockets directly mounted to the freehub body for want of a better description. but is still undergoing testing hence no firm release date yet. There's a lot more work than people imagine in sorting this, one of Hopes R&D engineers has spent nearly 6 months solid on the cassette already.