Realistically, the word 'hapkido' is an English phoneticism of 합기도, which is itself a Korean phoneticism of合氣道. I have always pronounced hapkido as hahp-kee-doe, a pronunciation that could easily be rendered as hopkido.
Considering that it is not an english word, any spelling that we have is phonetic. Unless we are using hangul or hanja, we are not spelling it correctly.
As a side note, the only time that I have ever seen it written 'hop' kido was on the Power Rangers, where they stated that Zack, the Black Ranger's martial art was "hip-hop-kido."
