I would guess you should have some screws down there on the corners of the PCB holding it down to the metal chassis.
Try to find and remove those if you have not done yet.
On the right side of the shield I can see some cables (brown and blue) with the noise suppressing ferret.
It looks to me that there is some space left to pull the board out if you can tilt it up and away from the power switch and main power connector.
BTW: If you have an incandescent light buld around, maybe from an old desk lamp.
Switch it on and use the pins of the power plug to cross link the heat sinks with the metal chassis and cross link ground and 12V from the outlets of the PSU.
Thats how I discharge the caps when working on such a device. Your way with the discharge resistor is basically the same, my light bulb however gives me some noitceable reply.
