Just a quick update: When filling the tank, I had a problem with foamy water/bacteria from my sand substrate and had to fill/drain the tank six or seven times. Needless to say, the foamcore caught a lot of drips and splashes over the back rim — but the only way I knew was that I could see a very faint darkening of the center of the board under the area protected by the tape. I could not feel any dampness anywhere, and there were no drips below the tank. (I have an open iron stand.)
Now that the water has cleared completely, I can see some faintly lighter streaks where I believe the water soaked into the front (tank) side of the posterboard/card stock. There are also some odd, darker squiggly lines where water may have been running between the glass and board.
I had to look closely to see all that, and it's not enough to bother me and will likely be obscured by plants/algae over time, so I have no plans to replace the foamcore. All in all, I'm pretty happy with its performance.