But your draft is about HTTP, isn't it ?
For HTTP you need URIs, or RFC 3986 reg-name,
not RFC 3987 ireg-name.
An HTTP header uses Latin-1. You could say
that clients are supposed to translate an
ireg-name into a reg-name as explained in
RFC 3987 3.1, that would be IDNA punycode,
not simply percent-encoded UTF-8.