I'm not all the way through the book, so don't spoil it. But I think a great megaproject would be to build a dwarven Math based on the great book by Neil Stephenson, Anathem.
In Anathem, the world is divided into secular and mathic, with the mathic portion consisting of cloistered monks living in monasteries called Maths.
The largest Maths have large clocks that are wound by ritual and accurate for thousands of years. The clocks control gates in a concentric series of walls that subdivide the Math. The Day gate, the Year gate, the Decinarian gate, the Centinarian gate, and the Millenial gate.
As you may infer, these gates open only once a day, year, 10-year, 100-year, or 1000-year cycle.
Dwarves, and goods, can only flow into and out of the cloistered sections every 1, 10, 100, or 1000 years. Quite the time scale for a fortress!
To simulate this, I would propose building trade depots inside of each section that are selectively blocked in such a way that only one is open at a time, the deepest one accessible.
Each section would need to be relatively self-sufficient... but I have a feeling that's not a problem for any of us. Many have built biodome-like fortresses walled off from any possible invasion.
Not all dwarves would be required to be 'monks' (or Avout, as Stephenson calls them). Indeed, a portion would live outside of the walls, simulating the seculars, living in relative chaos and poverty. These would create goods and items to 'trade' with dwarves inside of the math's Unarian area. This functionality would be accomplished through a stockpile with two entrances that toggle each day, set as a 'take from' stockpile that feeds into resource piles. (for example, trapped/hunted animals) and which itself takes from other stockpiles crafts and things. The seculars might then have their own trade depot (which would have to be blocked during one of the special opening times) to at least trade with the passing secular caravans.
On the topic of the clock. It is of course the greatest challenge of this megaproject, since the organization and shuffling around of dwarves between 1, 10, and 100 and 1000 year sections would probably be easily accomplished through simple re-assignment of beds.
I don't know what would be the best mechanism, but even in the book it is stated that the clock has regular maintenance and calibration, so it need not be perfectly able to keep time. If such a clock could use cauldrons of water fed by a river, each tipping and spilling a single 7/7 volume into its neighbor until we gained the resolution necessary (one day), then it might be possible to connect those to some sort of binary flip-flop matrix that then translated into a nice big 7-seg digital clock visible from above. (maybe filled with magma?!)
The act of 'winding' the clock, a daily ceremony performed in the book, may be a cauldron that fills up and needs to be pumped back up to the resevoir. A closed water system would be coolest, in my opinion. One that only needed occasional filling. However, this would require one huge resevoir, or some means of reducing the flow rate... I'm saying a lot without explaining.
Finally, the book mentions water power being used to operate the door, and also a frequent use of bells to chime different ceremonies. I would substitute fountains for bells and well, water power can still be used to open the doors, instead of the magic means of linking a pressure plate to the far away door.
Has anybody else read Anathem? Have ideas?