I'll send this proposal off to riplin:

4. Calculate "preH" = PBKDF2-HMAC-SHA512(key = salt, msg = passphrase, iterations = 10000, output_len = 64)
5. Calculate "strongH" = KDF(msg = preH[0:32], salt = preH[0:32], output_len = 64) This step can be outsourced to a 3rd party, if desired.
6. Calculate "H" = PBKDF2-HMAC-SHA512(msg = preH, salt = strongH, iterations = 1, output_len = len(root_key) + 32)


I've generated the relevant test vectors as well.