You assemble the D10A onto the flush mount first.
You see the outer groups of 3 holes? they are to install nut plates / anchor nuts into, and you attach the flush mount to the panel with screws from the front of the panel. Yes, you have to physically hold the unit up to the back of the panel while you screw / unscrew, and remove it out the rear like any other rear mounted instrument, but you never have to access the screws on the rear of the D10A.
I've done a few certified installs. Here's a picture showing the screws on the front of the panel.