> If I add non-trivial features to this SRFI, the set of available
> implementations drops from "all R6RS systems" to "none".  I am not eager
> for that to happen.

+1

> I will add flush as an optional argument to the
> make-custom-port procedures with a warning that it may not be called on
> all systems.

The most problematic functions in specs tend to follow this pattern :)

> I am considering adding the MIT Scheme feature of custom operations: a
> custom port is defined with an optional alist mapping operation names
> (arbitrary symbols) to procedures, and then there is a user-level
> (port-operation port name) that pulls procedures out of the alist.  But
> even that may not provide enough bang for the buck, as it has to be
> specified in such a way that it may return #f, just like flush.

Likewise. These effectively become optional features that look to the
user as if they are fully implemented everywhere (since they fail
silently if not). Please, let's take some more time to think about
whether this is a good idea.