I haven't been pushing this forwards mainly because I don't have the bandwidth myself to work on the implementation, but if someone else is able to do that, that would be great.
There are some details of the spec to iron out. One is a syntactic decision - whether to use a new keyword "mutable" to indicate mutable fields or not - I'll request comments on the proposal. There are other details of the spec to iron out related to the implementation, but I think it'll be easier to work through those by starting on a prototype.
Cheers
Simon