> The 'read' procedure that looks for the encoding declaration should be a
> special reader that's much simpler than the normal Scheme reader and
> handles encoding errors gracefully [...]

Here's a quick sketch of the idea implemented in 100 lines of R7RS
Scheme: <https://github.com/lassik/scheme-encoding-declaration>

It reads up to 1000 bytes from the file into a bytevector and reads the
first Scheme form from that bytevector, interpreting the bytes as an
ASCII superset. If there are any errors, it simply returns #f.