From my experience with the foam/rock wall, even coating the backside of the egg-crate with epoxy is not going to stop water from getting in, esp. if you "utilize" the wall for it's intended purpose; by this I mean, poke frag plugs into it to start cultures on the wall, etc... That is one source of the water getting in, the other source is simply the rocks attached to the egg-crate... I didn't coat my rockwork at all when I built the wall, only the foam portions, granted the edge of the rockwork did get coated with the epoxy but the rocks are porous, they have holes in them, etc.
The bottom line is; in order to completely water seal the wall would be extremely challenging to say the least, even coating the back with multiple coats of epoxy didn't fair too well.
Keep in mind this was a concern BEFORE I built the wall, and others (myself included) did the extra work try and minimize the water logging effect, but like I said, after 4 years of "being used" it simply became completely water logged even with the extra effort being applied. I am not saying it is impossible to make it completely water tight, but it would be very tedious and time consuming... plus, once you would start to add frag's and such, you might as well just throw out all that extra work... the water WILL get in as soon as you poke that plug into the wall...
That being said, the real Q really is how much does that affect the reef environment over time. As mentioned, I had amazing success with this tank, but, it only stayed up and running for 4 years and did eventually crash.