Well, for a header to get included in the Provisional Register you should
need two things:
(1) A specification published in an accessible place. That might be a web
site, an Internet Draft, or even an experimental RFC.
(2) A forum in which to discuss it. Which could be a mailing list or a
Usenet newsgroup.
I think this would be a good start. Though I'd strongly prefer that the
specification be published as an I-D (rather than say, an MS word document
on a web page somewhere), and that there be a permanent mailing list
for discussion of new header fields.
The other thing I'd like to see is some way of pushing back on dubious
extensions.
Keith