I'm pleased to advise this is now possible. I've tested in v6.3 and can confirm that the characters are passed to ResellerClub as entered by the customer. I made a registration attempt for teñsting.org, and this was the data passed to ResellerClub's API:

  • ...
  • [attr-name1] => idnLanguageCode
  • [attr-value1] => es
  • [domain-name] => xn--testing-5za.org
  • ...
  • [/co]
    In order for this to work you also need to pass the idnLanguageCode variable, so I've added instructions for doing that to our documentation: http://docs.whmcs.com/ResellerClub#IDN_Configuration