> Or saying “see section 6.12 of R7RS Small” the best we can do?

I think that is fine. Then people can look at it in whatever format they
want (HTML, PDF, Texinfo, etc.)

 > Of course, I am tempted to link to https://r7rs.aartaka.me, but I
don’t trust myself in keeping it up in posteriority, and I’d better have
some “official” source instead.

I would hyperlink it like

     <a href="https://r7rs.aartaka.me/#6.12">section 6.12 of R7RS Small</a>

If the website ever goes down, there is still the the text that tells
you what section it is, and someone can file an errata.

-- Peter McGoron