Does silicone adhere as well to epoxy as it does to glass? I don't know, but I would do a bunch of research beyond just asking the question here before I attempted this idea.
Silicone alone will effectively adhere a glass window to the interior of an epoxy-coated plywood tank, but in that case the water pressure is pressing the glass tightly to the wood. In the scenario you are proposing, the water pressure will be pushing the glass away from the glass/wood interface, so the silicone adhesion will be considerably more stressed.
Why re-invent the wheel? Glass is a known quantity in this application; wood/epoxy probably is not. Will a piece of plywood properly sealed with epoxy really save you that much money? I tend to think not.
Bear in mind that whichever way you go, you will need to remove all the interior beads of silicone from the whole tank before proceeding. Then, when you apply the rear panel, you will then need to re-apply that interior bead all the way around all the interior seams to be assured of a waterproof seal. Otherwise, the inability of old silicone to adhere to new silicone can cause leaks, even if the glass continues to be held securely in place. To clarify, I'm referring to the interior sealing bead only, not the structural silicone actually between the panes.
