The HTML-as-S-expressions format actually sounds very reasonable. Maybe
we should first investigate what has the best tooling (or is easiest to
write new tools for). Would something Scribble-like work? That would be
close to HTML-as-Sexprs. Even Texinfo could be a contender - tons of
other Scheme documentation is using it so we'll have to write some tools
to index and render it at some point anyway.
But if we don't get permission we'd probably have to start a new
cookbook from scratch at some point in order to give up-to-date advice
and cover more implementations. It'd be a shame if users had to keep
switching between two different cookbooks for the same job.