>     The question is, does it interfere with symbols if it's supported
>     without the #d prefix (e.g. 1_000_000).
>
> Not really.  RnRS doesn't allow symbols to begin with numeric digits
> (unless it is escaped), so it won't conflict.
> Gauche treating them as symbols is just an artifact of old Lisp
> convention, which is "word that can't be parsed as numbers are
> symbols"---besides, I was rather conservative to introduce "underscores
> in numbers" feature.  If we establish that convention I'll adapt Gauche
> to it.

That's great news :) It's good to have your opinion since you have
actually implemented the syntax.