They probably ought to be discussed in our docs :(

The only thing that may be encoded in an address is the "display name" (the first part returned by parseaddr).  (Actually the domain name could be IDNA encoded, but we don't support that directly in email.  3.3 will.)  So the easiest way to code this is probably to take a list of parseaddr'ed addresses, and append display_name as utf-8, and the address formatted as '<%s>,' as ascii.  Omitting the comma for the last one, of course.  Not very elegant, but I believe it should work.

If you want to get fancy you can split out the domain and run it through the IDNA codec to encode it before passing it in as part of the ASCII token.

Header puts spaces between ASCII and non-ASCII tokens automatically, so you don't have to add them to either the encoded or unencoded tokens.