My IDNA library now supports decoding IDNA strings via the to-unicode function:

(to-unicode "xn--mller-kva.example.com")
;; =>  "müller.example.com"

That’s in addition to the regular encoding for unicode domain names:

(to-ascii "müller.example.com")
;; => "xn--mller-kva.example.com"

Sadly, I haven’t managed to get the logic for case-sensitive punycode encoding to work yet. But fortunately, IDNA domain name encoding doesn’t require that! Anyone looking for some low-hanging fruit-shaped lisp projects is welcome to add that! (-: