Ok, did a bunch of testing with a friend. Zilean's timebombs use a similair mechanic, 1 point of true damage is done per tick of the time bomb to grab tower/minion agro in summoners rift. We tested both the time bomb and trundles pillar, and both correctly drew tower agro. We didn't notice any change in our hp values at all, we theorize this to be because the game instantly heals it off with your base hp regen.
On dominion, neither of them stopped a tower capture. Riot has either disabled the 1 true damage point on Dominion, taught the capping system to ignore the 1 point of true damage, or more likely has just remade the system over time to not rely on 1 point of true damage, and for the skills to just have an aggroing componenet built into them.
Another interesting note. I was unable to reproduce the capture stop with trundle moving them either out of or into range (I guess I suck at using pillar), but it appears to be only related to how far they were moved. My friend and I tested Janna's ult, and blowing someone across the point (they actually ended up closer to the point center at the end of the movement) and it completely stopped their capture.