I want to be able to simply designate a large rectangular channel area to be dug, and have my dwarves do it without any babysitting.
I think a pretty good solution to this would be easy to implement: For each channel-designated tile, use pathfinding to compute how many tiles it is from the designated tile to a passable non-channel-designated tile. Once you know the distance-to-a-floor of each designated tile, have the dwarves simply mine out the ones with the highest distance first.
This allows you to dig pits with minimal user control in many situations, including where there is only one access tile to the room with the pit in it (the door). You would simply designate, and then your dwarves will figure it out themselves.
You would probably have to rerun the distance-finding every time a new channeling designation is made or modified by the user, but it's a one-time cost, and not a big one.
The distance finding should be easy to implement and minimally taxing, CPU-wise. You could use a sort of flood fill where each tile which touches a floor is set to a distance of 1, and then its non-filled neighbors are set to 2 on the next pass, and so on. If the distance-finding only takes into account north/south/east/west moves, then this algorithm will have the nice side effect of keeping the miners from causing a cavein of isolated sections of designated floor.
This could also be applied to construction of walls/fortifications: the dwarves should build the innermost tiles first. It would also work very well for removing constructed floors.
One thing this will not solve, however, is dwarves being on the wrong side of, say, a moat. However, it would make digging large pits, even multilevel ones, much easier. I would very much like to see this in DF, as it would make digging my gigantic Moria-esque halls much, much easier.