>> If you need different code for each R6RS implementation, the convention
>> is to use one .sls file per implementation, with the implementation's
>> name in the filename: for example "mylib.chezscheme.sls" and
>> "mylib.loko.sls".
>
> Do the various R6RS implementation automatically look at the right files?

Yes, my impression is that they do. Each implementation has a particular
extension that you need to use exactly as it is written -- for Chez
Scheme ".chezscheme.sls", for Ikarus ".ikarus.sls", etc.

More generally, in all R6RS matters, when in doubt check how chez-srfi
and Akku do it :)