Yes, the buss bar you mentioned is labeled 120 VAC, but, at least in the U.S., there is no requirement to use it for that purpose. As the incoming hot lead can be connected directly to the bottom side of the main breaker, leaving that buss bar free, it can be used for any other purpose one might have. Unless there was a recent change, on the seven PD-4045's I have installed, I didn't have a ground buss bar to work with, so the unused bar worked just fine.
As I mentioned, this is also what the manufacturer gave the okey-dokey to do with that buss bar as well, as long as CSA compliance wasn't required.
BTW: I can't imagine installing one of these units without a way to turn power to converter OFF, that may even be illegal by most codes, and is unacceptable by my own code....LOL