it woul've been great if TI decided to use a 3V3 supervisor after the LDO, and not a 5V one on the USB / battery side.
They propably had a reason to do it that way, good engineering practice maybe. But it is not that hackable as it could be
This should work fine, unless you pick the wrong balancer port and put 8V4 in it instead of 4V2
The current draw of the launchpad is minimal compared to the current draw of the steppers, so there shouldn't be any unbalancing happening.
You should however add some kind of software voltage supervisor to switch the steppers off if the voltage drops below a treshold.