This topic is old, but I will reply if it is found through search functions:
I found a solution, what can be applied with small external hardware modifications. It is possible to take some measurements on HAT interface's pin "5VIN".
Measured voltages:
If powered by USB:
~4.7V
Powered by battery and switched off:
~0.7V
Powered by battery and switched on:
~2.4V
Using voltage divider with two same resistors, we get usable logic values for HIGH and LOW ( ~2.3V and ~1.2V). The measured values are not inside garanteed logic range but as a workaround: it works well. Maybe it helps someone else...