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.