I have to admit my last physics class was in the last millennium (and not even the last decade of that). But square wave DC sounds like an oxymoron.
I had a hard time finding detailed information on the 787 electrical system on Google. What I did find, though, seems to indicate that the +-270 V means that there are three conductors: one at ground potential, one at +270 V compared to that, and one at -270V. So, sort of like with the 120/240 VAC system common in the U.S., a specific piece of equipment can be connected to run at either 270 or 540 volts.