SRFI 146 already provides a guarantee of ordered immutable mappings (using (srfi 146 ordered) if supported by the implementation).
Well, that's just it: you can conform to SRFI 146 and provide only (srfi 146) and (srfi 146 hash). I'd like to see (srfi 146 ordered) a requirement and either leave (srfi 146 hash) optional, as it is today, or leave it out of the SRFI altogether.
There is already an asymmetry between SRFI 146 and SRFI 113, because SRFI 113 does not provide any ordered-only procedures.