Can I do an instrument rating checkride in an experimental aircraft?
Yes, but only if the examiner agrees. 61.45 says:


  (a) General. Except as provided in paragraph (a)(2) of this section or
  when permitted to accomplish the entire flight increment of the
  practical test in a flight simulator or a flight training device, an
  applicant for a certificate or rating issued under this part must
  furnish:
  
  (1) An aircraft of U.S. registry for each required test that—
  
  [...]
  
  (ii) Has a standard airworthiness certificate or special airworthiness
  certificate in the limited, primary, or light-sport category.
  
  (2) At the discretion of the examiner who administers the practical
  test, the applicant may furnish—
  
  (i) An aircraft that has an airworthiness certificate other than a
  standard airworthiness certificate or special airworthiness
  certificate in the limited, primary, or light-sport category, but that
  otherwise meets the requirements of paragraph (a)(1) of this section;


This article describes an IR checkride in a Van's RV6.