I have a Zwave fan controller - Leviton brand. Don't remember where I got it, but they do exist. It works well and has 3 speeds. I set the ceiling fan's switch at the lowest setting - meaning that when the zwave switch is on low the fan is actually running "ultra" low.
You might want to be careful with that. Many many years ago I used to work at a company that made ceiling fans so I learned lots about them. Anyway, a big part of the design of a fan is making sure that there is enough air-flow over the motor on the low speed, because if there isn't the fan can easily overheat. Overheating is never a problem at the high speed but it can be when its low. When you run it even slower, you are dropping the air-flow at a much faster rate than you are dropping the heat generated, so the heat-to-cooling ratio rises and a fan can overheat. I wouldn't recommend running it much lower than the low speed it was designed for. Anything higher than that shouldn't be a problem.