Alrighty then. So apparently from what I can gather looking at Dracula's code in CV3, setting any bit in a boss' attribute byte ($0470,X) other than bit 6 will render it invulnerable (also unable to hurt you). So if the boss is invisible, it can't hurt you. If the boss is frozen, it can't hurt you (although you can't freeze bosses to my knowledge). If the boss is ANYTHING except moving or brain-dead (i.e., no attributes set at all), the boss will be rendered ethereal. That also explains why not yet even once in Dracula's code do they ever tell his sprite to not be animated; the boss timer instead is used to animate him. I wonder if any of the bosses use bit 5 (inanimate); it's a popular bit with regular enemies.

On a side note, reading a boss's code is a pain in the ass. PISS OFF FAMICOM, I DON'T CARE ABOUT YOUR FUCKING CPU INTERRUPTS!