>I don't intend to use it to throw v6 at unsuspecting v4-only
>implementations.  "Don't do that, then."

I just wonder if you're spending your effort in the wrong place.  Yes,
you have some legitimate issues with PPP, but those are implementation
issues; they have nothing to do with the PPP protocol itself.  I'd
personally work on fixing the things in PPP that you don't like;
you could even interoperate with other PPP implementations, which would
be a bonus.  I can't really see anyone else in the known universe who
would spend any effort on v6/SLIP; that may or may not be an issue, of
course, but it's something to think about.

--Ken