Instead of detecting the battery drop, the spike can be reduced so the low battery detection becomes more stable.
In low power applications, 10mA or more power drain is usually related to burst events (such as wireless transmissions), which in turn generate spikes in power supply voltage. To cope with these burst currents, a bigger ceramic decoupling capacitor can be used.
This way you can assume 30mA are still OK for the BOR circuit if the bursts are short enough.
If you need to draw 30mA for a sustained period of time, you could use a buck-boost converter, although it will increase both cost and overall power consumption. I have to admit this sounds awkward to deal with a coin type battery, but applications can often surprise us
Jack, I agree that one should look at the current draw from the datasheet when coupling to such a low power energy source. However, expecting a processor to be able to check its own voltage is a mess waiting to happen. What I tell our software engineers - don't send in software to do a hardware job. BOR should ONLY be done by hardware. So if running off a coin cell, you need a processor that can safely run off of a very low voltage then choose a suitable reset chip to drive the reset pin of the processor. The min voltage of reset should be just above the min operating point of the processor. The idea, of course, is to put the CPU into reset just BEFORE the CPU loses its mind.