Ran "adb reboot bootloader" now can't boot past logo (Azpen A746)
Fully draining the battery will unbrick an Azpen A746HD in this situation.

I did further testing and was able to repeat the conditions for bricking and unbricking the tablet:


Running adb reboot bootloader will render the tablet unbootable as described in the original question (hard-brick).
Fully draining the battery will clear out whatever issue was preventing boot, unbricking the tablet.