Sometimes following up on a bug takes you a lot further than you expected. Debian bug #337041 looked like it was going to be fairly straightforward once I upgraded coreutils to figure out what the new IUTF8 flag actually did, since the SSH2 protocol already supports transferring termios flags around.
Unfortunately, since IUTF8 is relatively new, it doesn’t have a number assigned in the draft connection protocol Moreover, that Internet-Draft is in the last stages before becoming an RFC and can’t be modified any more, and it doesn’t include any facility for private-use extensions. D’oh. To add further complication, since IUTF8 is Linux-specific, it’s not hard to imagine that some other OS might introduce something with the same name but subtly different semantics, and so the SSH protocols can’t just defer to POSIX for the definition but instead have to spell out exactly what they mean.
As a result of all of this, it looks like the best way to make progress might be for me to write an I-D myself that creates a channel extension to set or clear IUTF8, and attempt to enlist support from some upstream implementors. I didn’t expect bug triage to lead me into the Internet standardisation process quite so quickly!