> r7rs-small defines some, for example exact-complex. Maybe I'm just
> missing something.

It says those are just suggestions:

> Here is an example of what features might return:
>
> (features) =>
>   (r7rs ratios exact-complex full-unicode
>    gnu-linux little-endian
>    fantastic-scheme
>    fantastic-scheme-1.0
>    space-ship-control-system)

Though it's good that it sets a precedent :)