You could run the bow speakers off source unit power if two HU channels are otherwise dormant, but I do not recommend it.
The cockpit occupants are not going to hear the bow speakers which are on the opposite side of the windshield so you need just enough power for the occupants in that small bow area. Is this going to be a getaway for children or adults wanting to have conversation, or is this area needing to be competitive in power?
HU power, if rated the same way as a good external amplifier, is no more than 15 watts per channel. That will normally not be enough when underway.
So if the bow speakers are expected to compete with movement, wind and the cockpit system, with HU power only, then one of two scenarios will play out. First, the bow HU power will fall flat on its face prematurely. Second, if you gain the other externally amplified zones inordinately high so that the bow doesn't clip way early, then the bow will lag behind at all listening levels and the rest of the system will have an inordinately high noise floor.
So again, typically if the other zones are externally amplified then the bow must follow suit.