The PoE hat has a built-in cooling fan, and even has drivers in Raspbian to automatically turn it on and off based on the CPU temperature.
However, any guides to add a temperature-controlled fan seem to use a Python script and rc.local to handle this behavior, duplicating the existing system. Moreover, I'm not sure how well the third-party scripts will hold up due to future updates, and it's surely less efficient. I'd like to use something as close to the preinstalled drivers as possible, rather than reinvent the wheel, but the Hat is expensive and I don't have any need for (or have, period) a PoE system.
Is there a "fan-only" hat that uses these features, or a way to build one? Looking at the PoE hat, it uses some kind of Atmel chip, so possibly I could try to get and program one...
On the other hand, what is it in Raspbian that adds the fan-control drivers for the PoE hat? Is there some kind of package or set of scripts (say, a fan-control shell script and an init file to daemonize it) that I could add to a non-Raspbian (I have a fondness for Ubuntu, although Raspbian is improving) system, or adapt to a different fan control method, if the I2C Atmel thing is not possible?