The funny thing was that I was going to post the same thing last night. But it's been a while since I've competed (or coached anyone to compete) so I thought I'd look it up in the WTF rules and quote the article/passage. And guess what... It's now allowed.
http://www.wtf.org/wtf_eng/site/rules/file/Competition_Rules_(Tashkent_version_final_2010).pdf
Whereas the rules used to state* "Fist technique: Delivering techniques by using the front parts of the forefinger and middle finger of the tightly clenched fist", now Article 11 (page 19 of the link above) now describes permitted techniques as "Fist technique: Delivering a punch using the tightly clenched fist" with the explanatory note "In the original Korean terminology of techniques, the term “Pa-run-ju-mok” can be interpreted as a correctly clenched fist. Therefore, striking with the correctly clenched fist is permitted without consideration of the angle, trajectory, or fist placement of the strike".
The fist must be closed, but hammerfist or backfist are not just permitted, but scoring techniques under current WTF rules.
That said, IRs may have been told slightly differently so you may have more current knowledge (if you are an IR) but going by "the letter of the law", they are now valid.
* The old 2006 rules are available at http://www.britishtaekwondo.org.uk/usrdata/File/downloads/wtf_competition_rules_english.pdf
I started a new thread instead going further off topic.
What I was told was that backfist will not score. IRs are being instructed to specifically look for cover punch as the scoring punch in matches. The only other situation in which a punch will score is if the punch has tremendous power such that anyone can recognize that it was a scoring punch. This would include stuff like a jumping in punch or even a hook punch which causes the opponent to buckle from the shot.
But no backfist.