Currently the stability policy says:

Updates to this specification will only reserve, define, or require function names or function option names consisting of characters in the ranges a-z, A-Z, and 0-9. All other names in these categories are reserved for the use of implementations or users.

It doesn't say option values are restricted. If we expand the stability policy, it should not encompass values that are literals--it should only restrict enumerated keywords (like those proposes for :currency's currencyDisplay option).