Is there an intended use case that necessitates the inclusion for the :|| syntax? The SRFI says that most implementations do not offer a syntax. I think ":||" will be a bit annoying to implement in a reader: it isn't prefixed by "#", and ":" is a valid character in identifiers.
I would also like to echo John/Shiro/Marc's concerns, and add another comment: why would uninterned symbols need to be specified across implementations as a standardized SRFI? Implementations that already have uninterned symbols do not need the SRFI. Implementations that don't have uninterned symbols do not have a compelling reason to add them. Extended macro features are interesting, but they could be developed as part of a standalone macro-expander (plus reader with extended syntax) library rather than a non-portable SRFI.
