Constantly engaged disc brakes, how hydraulic brakes retract?
Hydraulic brake pistons are actually retracted by the rubber seals. The seals are deformed slightly as the piston advances, then they re-form to provide the retraction force. This works because the retraction distance is actually very small. The seals design also allows piston advancement to take up pad wear. As the pads wear the piston moves slightly beyond the deformation limit of the seal and slips through it a small distance.  

There is a good Park Tool video that shows how this works.

I would definitely be concerned that holding the brake on for a long time would affect the rubber seal. What might happen is the seal will slip back on the piston while it is pressed up against the rotor then not retract it properly.