It would be great if f.lux could call a custom command (eg run a bash script) when the color temp is being transitioned at sunset/sunrise.

I would think that f.lux would call the command in the following way:
custom_command current_temp new_temp transition_time
where:
custom_command = the command line the user has specified - eg ~/my_awesome_script.sh
current_temp = the current color temperature, in K
new_temp = the new color temp that f.lux is transitioning to
transition_time = the duration of the transition from current temp to new temp, in milliseconds (or floating point seconds perhaps)

(this feature request originally posted here as part of a larger post: https://forum.justgetflux.com/topic/2604/diy-lights-with-osx-f-lux/1):

Thanks!