>> a standardized S-expression file that each implementation could put in its
>> source release to tell things like what SRFIs are supported
>
> I think this is not very useful.  In CHICKEN, for example, a large number
> of SRFIs are implemented as eggs (add-on libraries), often by third party
> uthors (ie, people not working on the core system).

That's a good point. I didn't realize that. So the implemented SRFIs
would be a union of the implementation's official release and package
manager(s). (Also it's conceivable that an implementation would only
support a SRFI on a particular OS or with optional build flags.)

I scraped together a list of all the packages in Akku, Chicken 4 and 5,
Gauche, Guile and Snow-Fort here: <http://lassi.io/temp/packhack.html>.
This could be turned into S-expressions. I don't know if there would be
a reliable way to include information about implemented SRFIs in there.
Maybe there could be a standardized package metadata file. It ought to
be designed by people who have a lot more experience with Scheme than I do.