Yes. 12v should work on VUSB. I still don’t like how this is never stated explicitly in the docs. You would have to review the Boron schematic on your own and look at how the PMIC (BQ24195L) is connected to the board pins. Then you have to pull the BQ24195 datasheet and look at what that maximum spec is.
Here you can see that VUSB labeled on the board is VIN on the schematic:
Here you can see that Vin is directly connected to the BQ24195L VBUS pin:
Normal operating voltage for the BQ24195 is 3.9 to 17 volts. Maximum rating is -2 to 22 volts. The capacitors on Vin are spec’ed at 25v and it looks like the VBus diode can handle 30V. So in conclusion, you should have no problem powering the Boron by 12V DC. You may even get away with up to 17V or so… but at your own risk.
EDIT: @RWB makes an excellent point below, the higher the input voltage, the more heat that is generated. You must take great care as to not overheat the board when using out-of-spec voltages which is probably why it is only recommended up to 12V.