When you play by the rules, you are limited to your abilities. But if you can squeeze advantages here and there, you can climb some extra spots, maybe to the top.
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! :)
Before the e-hogus a fellow competitor used to punch in a way that was not damage effective, but made a lot of noise and looked like a real punch (almost like a backhand slap), scoring a lot of points. Their opponents claimed that they weren't even hit. Another guy used to launch kicks in the last possible moment of a clinch (almost always a jump-turning-and-back-kick, tuio mondollyo tuit chagi, don't know the name in english), just when he noticed in the corner of the eye the referee intended to break the clinch.
Just one example:
He noticed one ref gave verbal warnings only the first two times someone stepped out of bounds, so he had me bait the opponent into throwing a bunch of kicks as I retreated until I had two verbal warnings. My opponent was rather pissed off before I threw a single kick, upped the aggression and fell right into a counter.
He was good at all the regular tactics too; my only TKO was accomplished by starting out round 2 with exactly what he told me to do.
Re: the hogu thing, I did see someone do a push-kick on the hogu, then drag the foot down, pulling the hogu with it until there was no padding over the collar bone and punch there. He broke two people's collar bones and the ref was oblivious. The second opponent retaliated and it escalated with the match ending -3 to -2 (yes negative for both people). The guy who initiated the dirty stuff won, but had a broken instep (his opponent caught a roundhouse by raising his thigh and dropped an elbow directly onto the instep).