I chatted with the Fully Kiosk Developer, Alexey, and he is adding an option called 'Keep Screen On while in Fullscreen Mode' in the upcoming Fully Kiosk Browser 1.27 release which you can disable to get the screensaver to work when a dashboard is in fullscreen. 
I've tested a beta release of this and it seems to work well. Once Fully Kiosk Browser 1.27 is released, you'll find the new setting under 'Advanced Web Settings'
