I would just wait until you have the car back on the road, unless you offer the spoiler up to the hatch and mark it off in the best position before you remove the hatch, so that the body shop knows what position to fit the spoiler.
The hatch is far to heavy for your plan and there is also the chance of breaking the glass and also damaging the paintwork to the car when refitting.
Your body shop should turn this round in a couple of days.