So I'm not convinced that 6598 space should be treated like 1918 space. Specifically, the second paragraph of the rfc states:

   Shared Address Space is distinct from RFC 1918 private address space
   because it is intended for use on Service Provider networks.
   However, it may be used in a manner similar to RFC 1918 private
   address space on routing equipment that is able to do address
   translation across router interfaces when the addresses are identical
   on two different interfaces.  Details are provided in the text of
   this document.

which I read as, "It's not private like rfc1918 space, but sometimes certain people can treat it similarly." Are there more convincing arguments for treating 6598 like 1918?