I know that with the direction the sport has taken in the last decade or so, there are probably a hell lot of hacks right now. I even heard that players would change tactics (ie., rely on certain kicks/strikes more) depending on the hogu brand in use.
But I competed before the advent of e-hogus and the one trick I found is punching just at the upper edge of the chest part of the hogu. That's where the rubber padding ends which transitions to the mere "cotton" padding that fluffs the straps. This meant that my opponent might actually hurt from my punch, while plainly making contact with the protective gear. In an era where punches rarely scored points if at all, this was useful. I felt like I could punish people for clinching too much.
(Needless to say, this was in high school competitions. I dunno if it would've been as useful in higher tiers.)
So did you find anything else? I'm curious! :)