%NN encoding, request/response headers, UTF-8 ?

I'm curious 
 - whether Unicode characters with values 0x100 and greater are allowed in 
   request headers (especially the request line)
 - if so, if UTF-8 encoding is allowed
 - how a client indicates to the server that it's using UTF-8
 - how an HTTP server application decides how to interpret
   hex-encoded information, e.g. is %C3%B1 encoding two characters,
   or the UTF-8 encoding for the single character "ñ"
 - how/if a server might use UTF-8 in its response headers

It looks like any content that is sent with MIME headers (e.g., an object 
sent by the HTTP server) could be announced with a charset value indicating 
UTF-8 encoding, but that headers (request or response) are only expected to 
contain characters 0x00 -> 0xFF. Yet I don't see this clearly stated.

It seems fairly clear, though, that double-byte character sets (e.g., 16
bits for each character regardless of its value) should not be used in
either request or response headers. Right?

I appreciate any light you may be able to shed on this topic.

-Peter
http://www.tux.org/~peterw/