The current draft gives a comprehensive read syntax, but no facility to
print numbers with underscores.

I think it would make sense if the printer used underscores too. But
that should probably opt-in rather than opt-out, so that existing
programs print using standard Scheme syntax by default. What do you
think? BTW what's the usual way to give settings to the printer in
Scheme - is it done using 'parameterize'?