I replied on the GSoC proposal itself, but since it was cross-posted to the mailing list, I'll include my comments here too:

I've reviewed this proposal, and I think it outlines a step in a good direction. As I've discussed with Duncan in the past, my biggest concern around a system like this is the complexity of the UI for a developer; having to juggle different environments just to get basic work done could be a significant hurdle. As long as attention is paid to ensuring no "UI regression" for simple cases, I think this is worth pursuing.