Defining extensions for the "Expect" header

Hi,

RFC2616, section 3.20 
(<http://greenbytes.de/tech/webdav/rfc2616.html#header.expect>) defines 
the "Expect" header with extensibility in mind, but doesn't mention any 
registration process.

Is this an oversight? Who can legitimately define new extensions (did 
this happen so far?)? I'd assume that an IETF standards track document 
can do this (in the same way as it already occurs for method names and 
request/response headers).

Best regards, Julian