For the Archived-At header, defined in http://www.ietf.org/rfc/rfc5064.txt,
which is related to list headers but somewhat different because it
indicates per-message archival location, I took a forward-looking
approach and included this:
2.4. Syntax Extension for Internationalized Message Headers
There are some efforts to allow non-ASCII text directly in message
header field bodies. In such contexts, the URI non-terminal in the
syntax defined in Section 2.1 is to be replaced by an
Internationalized Resource Identifier (IRI) as defined in [RFC3987].
The specifics of the actual octet encoding of the IRI will follow the
rules for general direct encoding of non-ASCII text. For conversion
between IRIs and URIs, the procedures defined in [RFC3987] are to be
applied.
My understanding is that this is enough for people to figure out
that with EAI, they can use IRIs (encoded in UTF-8) in the
Archived-At header(s) of EAI messages. If somebody thinks that's
not enough, and a revision of RFC 5064 is needed, please tell me.