Steve the folks at Proline and I disagree on how the floor framing should be done. I have installed many of their drains and have developed my own approach.
Your idea of the 1/4" backer board is a good one but you need to remember that the install should be "Tile Ready" before proceeding.
This bathroom shows Wonder Board Lite over 5/8" sheathing....
We often double down on the plywood and add concrete board to the system.
That and Simpson Strong Tie hardware and extra 2"x4" blocking and 2"x6" blocking below the subfloor to remove deflection.
The Proline installation technique outlining removing a section of subfloor is a scary one in my opinion. Beef up the subfloor. You do not want the floor flexing at your drain connection.
Call me if you need a little more heads up. JW 604-506-6792