Firstly, I've been blocked by the lack of a feature like this in the past and will be very happy to see something along these lines come to fruition.
Since the eventual full reverse-generic syntax has been referenced here a number of times I wanted to reference this post A new idea about generics that I made earlier this summer. The reason I think it may be relevant for some who are interested is because the syntax I propose here is at odds with the hypothetical reverse-generic syntax that you use in the post @bdriscoll, which as far as I've seen is the most commonly referenced potential future-syntax for that feature. I know that this syntax extension that you're proposing here does not necessarily step on the toes of that bigger decision (about the design of the full reverse-generics syntax), but maybe it does, so I thought that this discussion that we had at the post linked above might be of interest to someone.