I hate the idea of it just draining down until the battery protection circuits kick in even when the device isn't being used.
What does "isn't being used" mean? Processor asleep? In Power Down? On / off switch set to off?
How much wall clock time do you expect to get out of the 400mAh battery? (How many minutes, hours, days before recharging?)
If you make the total resistance across the voltage divider ~10K ohms, the maximum current is 0.42 milliamperes. In theory, draining to half the battery's capacity, that's 19 days eaten by the voltage divider. Too much?
I assume you could add a transistor (MOSFET) to the circuit to separate the processor from the battery and eliminate almost all of the drain. (A "hardware head" will have to advise you.)