Thanks for the response, @pkunk, it turned out to be the display end of the display cable!!
I had already tried re-seating the mainboard end of the cable, to no avail. Popped off the display as per your suggestion and saw this monstrosity:
Re-seated like so:
and now we’re cooking! There’s nothing quite like the euphoria of seeing a BIOS error screen on a DIY build… 
Thanks again!