I suggest store the total battery voltage at shutdown in eeprom and then use that at next startup until all the CAN readings are in.I still have a goodly amount of software to write. The BCM replacer has a boot up issue as it takes a while to get all the CAN data so the MCM throws a hissy fit at being told 'no volts!' over BATSCI. I now have a second board so I can do some work off the car and have a permanent on car test rig.
You can use an IGN switched circuit to tell your teensy board when to store the data.
I presume it is powered from the 12V IGN hold line or from the MCM 5V bus which stays on for about 5 seconds after the ignition is switched off.