>     For implementations that don't support it, skipping the thing is a
>     one-line macro:
>
> That works in a file of plain code, but in a library file, which
> typically has just a library or define-library form, it won't.

`.sld` files can probably be written in ASCII or UTF-8, and any other
declarations they need could be extensions of the define-library form
instead of a separate declare-file (or should it be define-file?) form.

I don't know if there are cases where the declare-file would need to be
the second or third form in a file, or nested inside the (library) form.
Can a R6RS library file have other stuff before the (library ...) form?

> But overall I think this approach is beating the air.  UTF-8 is so
> dominant

I agree and it's a good thing. My points are:

1) Some Schemes already parse encoding comments anyway.
2) The declare-file form could have other stuff too, e.g. copyright.
    Basically anything that you'd otherwise invent a magic comment for.