Good point, and I have the same view, being aware of it.
From speaking to people I've actually met (we had Prius owner 'meets' in the early days), a couple of people who actually did it (on original Mk 1 Prius) had no problems, because it was obvious (red triangle and other warning lights) and they pulled over pretty promptly (using only a little HV battery juice). I believe it needed 2+ gallons to persuade the system to restart.
I've heard less reliable reports that later models didn't highlight it, and weren't aware they'd run out until the car stopped with a badly depleted HV battery, that wouldn't restart the car, or at best needed something to be reset by a dealer (or maybe these days someone with a Techstream or similar). I must emphasise though, I can't vouch for this.
In the early days, a lot of us regarded it as a bit of a challenge to see who could go the furthest after the low fuel warning without running out, but since the stories of later cars (which may or may not have substance) I don't play that game any more.