Will this circuit suffice for low battery detection for any microcontroller?
This circuit won't work - the transistor will be saturated, and the GPIO pin held low, until the battery voltage drops to ~ 0.7 volts.

Also, your thought of using an LM7805 to regulate your 7.2 volt battery down to 5 volts will only work with fully-charged batteries, as the 7805 need a minimum of 2 volts across it to maintain regulation.