Using 555 as an external Watchdog
ok guys - found the answer: this post here at electronics-stackexchange gave the right hint: Watch-Dog Timer. And my gut feeling was right as well - i simply use a NPN-Mosfet that is being triggered by an Arduino-PIN (which easily can be done while sleeping as well as sleeping means waking up every 8seconds anyway) that connects the capacitor to ground and therefore discharges the guy. Awesome. 
This solution can be used as a real fallback in case of whatever happens, while operating the system with built in watchdog-functions while everything is ok.