My only real Boeing experience is the 737 and I am working from the Smart Cockpit ELEC pdf for the 757 BUT the HBB is, in my experience, powered only by the battery via diodes - and we have had a thread on this not too long ago.
The 757 may be different, but I have not seen an HBB powered from a main DC Bus except for the Hyd gen on the 757.
Over to 757 experts?