That's actually extremely plausible.

A smart UPS needs to have access to some very low level functionality on the machine it's connected to in order to safely power it off in the event of a power failure. So you're talking about having at least some root / kernel level access there. It's not crazy to imagine that a malicious driver update could slip in more.

As another poster commented, I'd like to see this actually simulated to be sure, but it's not an idea that I'm willing to dismiss out of hand.