Is F.lux open source software? I don't see a link to source code on their site and a quick web search didn't turn anything up. I wonder why they would lock down development when they could get a lot of great community support (not much to lose; it's free and they just ask for a $5 donation).
Unfortunately not. However, there is an open-source project Redshift[1] that I'm sure would be greatly welcoming of community support :). It also has a GTK client, etc.