Recently, while playing as One Fist, an issue came up... having a certain skill which I can't remember the name of, but allowing me to Battle and place a Guard, I began to wonder... does the additional hook attack apply on an interrupt as well? Because if so, that's 5 attacks, which is pretty powerful, especially for a hero with only one hand...

Since his ability doesn´t say "+1 hook attack to any attack" I would say clearly "no".

It says "+1 hook attack during any action" (i.e. Battle, Run, Ready, Advance) and executing your Guard order is not an action, but a part of the Ready action from his turn (and he already got his +1 hook attack from that).

One-Fist does not get to make an extra hook attack during a guard order. His special attack only occurs once during his own turn.

Knight + Unmovable + One-fist = 3 attacks, 1 hook attack, 1 guard attack, 2 mp's

A combination (ab)usable by anyone, really.