We've been discussing this further downstream in the Fedora Python SIG, and we have a draft approach that we're pretty sure will work for us (based in turn on the approach Armin Ronacher came up with for click), and we think it should work for other distros as well (as long as they already ship the C.UTF-8 locale, and if they don't, they should fix that limitation anyway).

So I'm assigning this to myself as I think the next step will be to write a PEP that both proposes the specific idea as the default behaviour in 3.7, and also encourages distros to opt-in to trialling it as a downstream patch for 3.6.