Those ideas for the clock sound immeasurably more dwarfy than mine, well played, sirs.
As for the function of the clock, i think a doomsday device may be a little bit lame.I personally would set up a foyer of some kind, with caged bait and such to lure in invaders and siegers. every 100/200/whatever years, a drawbridge would open, letting the mass of enemies collide with a dwarven army given hundreds of years to prepare.
Also, if anyone here has read the book "Anathem" one could design a more domestic system based around those ideals. The fortress would be divided into 3 sections, one section would be opened for 1 month every year, allowing outside resource gathering. The next section would open for 1 month every 10 years, and would be much more dwarfy, doing mining or crafting or something, and the third would open one month every hundred years, for the dwarfiest of the dwarfs, master adamantine crafters and mechanics. If you wanted to follow the book all the way, one would build a completely seperate, self sufficient fortress on top of the mountain the rest is built in to, linked to the clock mechanism, triggered to open every thousand years. Personally, i would militarize this fort and at 1k i would have an organized extermination of the other three sections.
Oh, and the clock must be visible from all three, without allowing the three to mingle outside of gate-open time