Just a little more...
I know I could do this same thing by using a motion sensor (MS16A for example) and some kind of dimming switch but I thought it might be nice to keep this self contained and not use the HA system. I found a couple Lutron modules that might do something interesting (they claim dimming, motion sensing of various intensities, light level sensing, etc.), but I don't know if they would do what I want and they look to be $150+ (ouch!).
If I can't find a simple single gang module, I'll probably just keep what I have (dimming rotary switch) or replace it with a toggle style dimmer switch. I didn't think this type of thing would be difficult to find, but it is starting to look like it doesn't exist.