I came to the same conclusion you did, for almost exactly the same reasons.
This may be stating the obvious, but I'd go further and say that any
such extension should ONLY define exactly what \x character sequences
are overridden, and leave all other \x character sequences alone. This
would potentially allow multiple such extensions to coexist.
I'm not sure I follow this CON that you list:
- escapes only usable in quoted strings
Why can't the extension also be usable in multi-line strings?
As for this question:
- does there need to be a registry for the redefinitions
to prevent conflicts between such extensions?
I'd say "no". The published RFC spec should be a sufficient registration.
Philip Guenther wrote:
2) change the base spec to say the \x maps to x unless overriden
by an extension; extensions may redefine any \x except \\ and
\". Scripts SHOULD NOT contain extraneous escapes. Then, create
an extension which defines \xFF, \uXXXX, etc
I therefore would prefer (2).