Update: I drilled a 2.5mm hole in the mount, as shown below. I first disassembled it (removed the spring and separated the two halves). The hole is centred 2.0mm from the corner of the notch at 45 degrees.
The bolt head is now accessible with the hex key even with the DIN mount assembled in place. Just remember to insert the M2 bolt before putting the DIN mount on.
0_1592481851722_din-mount.png