In the two-CFIs logging tie thread, Chris Ford pointed out that a case that I linked has some language about this issue. Couldn't find the reference at first, but eventually did.
The interesting thing about this is that in the case, the FAA was arguing the opposite. The Eastern regional counsel just a year ago said the opposite. And within the past year, a CFI was charged based on the opposite (although the charges were dropped ). So, in combination with everything else, it looks like the FAA still has trouble with this one.
The source is FAA Order 8700.1 - General Aviation Operations Inspector's Handbook, Volume 2, Chapter 11, Section 1, Paragraph 15. This is the entire paragraph"
==============================
15. Regulatory Requirements According to FAR 61.195(b), a flight instructor may not conduct flight instruction in any aircraft for which he or she does not hold category, class, and type ratings, if appropriate, on the pilot and flight instructor certificates. The phrase "if appropriate" applies equally to and in combination with both certificates when instrument instructor ratings are involved.
A. Single and/or Multiengine Ratings. According to FAR Part 61, flight instructors who hold an "instrument - airplane" rating only on their flight instructor certificate are authorized to give instrument flight instruction in single and/or multiengine airplanes for instrument certification provided they hold single and/or multiengine ratings on their pilot certificate.
B. Class Ratings. Flight instructors who hold flight instructor certificates issued under FAR Part 61, which allow only instrument instructor privileges in airplanes, may give instrument flight instruction in any class airplane that is listed without restriction on their pilot certificate. Instructors holding only a helicopter - instrument rating on their flight instructor certificate are limited to conducting instrument flight instruction in helicopters.
C. Ratings Limited to Instrument. Instructors with ratings limited to instrument may not give instrument flight instruction to students who do not hold category and class ratings in the aircraft used, since this would be instruction for the addition of a rating that conveys other than instrument privileges. These instructors may not certify logbooks or recommend applicants for any aircraft category or class rating.
==============================
Caveat about this Order - it's a guide for FAA inspectors and has a tendency to be out of date. It still contains references to taking the ATP checkride before the 23rd birthday, something the FAA specifically got rid of in the regs 8 years ago.