There is a Take Off Config warning if the Park Brake is set to park when either thrust lever is set to FLX/MCT or TOGA. Other than that, the only annunciation in the air (e.g. if someone accidentally set it to ON in flight), is an amber "PARK BRK" memo on the EWD. This has happened - a crew were "fiddling" apparently and set the brake to ON in flight. When they released it, the BPTI still indicated full pressure. Numerous things were tried, in the end as I understand it, the pressure bled off by itself and an uneventful landing was made. We had a notice sent out about it, asking us not to fiddle with the park brake in flight.
As for "an accident waiting to happen" - I've been on the bus 4 years and never heard of it happening accidentally. It was only that one time, which was a classic case of "what happens when I do this?"
