> We need a way to include
> implementations that delegate away the responsibility of implementing
> SRFIs to third-party packages (eg Chez w/ Thunderchez)

All Schemes do that (in the sense that for every Scheme implementation,
you can find some SRFIs whose sample implementation works in that
Scheme, yet that Scheme doesn't ship with built-in support for that SRFI).

> as well as
> implementations that solely depend on portable reference implementations
> of SRFIs (eg TinyScheme).

How about the good old "traffic lights" system:

red - this SRFI is not known to be available on this Scheme

yellow - this SRFI is known to on this Scheme via a third-party package

green - an implementation of this SRFI comes bundled with this Scheme

Someone with an eye for color could tweak these colors to be more
pleasant, but that'd be the basic idea.