Beaglebone Black Power Switch Detection
Your approach of monitoring the VDD_5V going into the AC pin of the PMIC seems OK. There is also the possibility of reading the PMIC Interrupt registers or Status register. Checkout chapter 9.6.4 and forward of the PMIC TPS65217 on TIs website LINK. According to the description in table 3 and table 11 the AC pin power level should be possible to readout. See the Status register description here: 

  

If you need the system to shut down really fast, you probably want to utilize the interrupt register.