The A320 Park Brake can definitely be selected ON during flight.
ECAM will display PARK BRAKE ON in red as a reminder that this is not a good configuration for landing.
The only real danger playing with Park Brake handle in flight is that after the selection of gear down the Park Brake Control Valve may be stuck in the open position or the handle may still be giving ON orders.....
On the ground I have personally experienced the A320 Park Brake Control Valve sticking in the OPEN position. It could only be released by a powerful blow with a wrench by engineering.