> Gauche has similar concept as port-flag (we call it port-attribute).

+1

It's useful to permit attributes that are not flags (boolean values) -
e.g. the Common Lisp reader keeps track of the current number radix.

"Property" is probably a more common word than "attribute" for things
like this but I don't know for sure.

> It is also useful to write, for example, "bytevector output port"
> similar to r7rs string output port

+1

IIRC some Schemes have one already.

> I agree that it doesn't need to be in this srfi.

OTOH it would be nice to fit all custom port stuff into one SRFI.