I'd think about screws from the back side through the plywood and into the screw holes in the Fastrack pieces. Viewing a Fastrack piece from the backside this is what you see:
The screwdriver tip points to the mounting hole.
I would first attach the track to the plywood in the conventional manner, driving screws through the Fastrack mounting holes and through the ¼" plywood. I would then stand the whole thing vertically to provide access to both sides. Next I would remove one screw at a time from the track side and drive a slightly larger screw through the hole on the back side of the plywood and into the Fastrack mounting hole.
Oh, and I find ¼" plywood to be just fine for decking. In my case I simply drive screws through the plywood because the underside is open, unlike your situation wherein the floor will be immediately adjacent, thus the need for no protruding screws.