I'm going to stick with the -> convention for SRFI 189 at least, with explanatory text like this:
This SRFI supports converting Maybes and Eithers to other Scheme objects. It also provides for converting the usual protocol for using Maybe and Either to alternative protocols historically used in Scheme (and other Lisps). An example would be the protocol of returning a specific true value on success and #f on failure. The notation "->" is used for both kinds of conversions.
I haven't read 195 in enough detail to know if it's appropriate there.