Well, both Gen. Choi and, I believe, GM Hwang Kee both stated they learned Taekkyon to various degrees (Gen. Choi said his knowledge was pretty rudimentary; I don't know if Hwang Kee claimed to "master" Taekkyon or not). The important thing to recall about Taekkyon was that it was more of a folk game than a martial art in any sense that we would think. People make a big deal about there not being any "record" of people such as Gen. Choi and GM Hwang studying Taekkyon but that's a bit like saying there's no record of me playing kick ball when I was a kid. As far as I can tell the importance of rank, certification, etc. in Korean martial arts really is a result of the Japanese influence on Korean culture not only through the occupation but also from the Kwan founders' time spent studying in Japan.
That all being said, I have no problem saying that there is some influence of Taekkyon on Taekwon-Do. In the ITF there are a few foot techniques that resemble those found in Taekkyon more than in karate, for instance. I would also say that the way the body is moved has much more in common with Taekkyon than karate with the rythmic raising and sinking of the body. When I trained at a KKW school this was absent and, in fact, a greater emphasis was placed on keeping the head level a la karate (how common this is now I don't know).
Does anyone know of any other Kwan founders (or early Kwan Jang) that claimed to have studied Taekkyon? I believe GM Kim Soo did, but he isn't really a Kwan founder or Kwan Jang (though he did instruct at the Chang Moo Kwan for a time before moving to the Kwang Duk Won).
Pax,
Chris