I applaud your efforts to engineer an intertidal zone but I'm really sort of stuck on what animals in the intertidal zone you wish to showcase? All the things I can think of in that zone, are maybe things that could easily do without the low tide (or could just have a section of land/rock in the tank that is above the waterline, like a paludarium of sorts, where the in-betweeners could go if they so desired).
Any fish, mollusk, starfish, etc. that can tolerate being in an intertidal zone can likely do just fine without being exposed at low tide. The only exception I can think of are those little beach crabs.. And I dunno, I think a beach area in the tank could probably service them just as well as anything (and could be an interesting feature point of the tank as well).
Again, it's something interesting to engineer, I've often thought about ways to accomplish it as well, but in the end I wonder if it's really necessary, i.e., if you could enjoy a coldwater habitat without the varying water levels of a tide.
Another method is to have two overflows, and an actuated ball valve on the lower overflow. Dial back the lower overflow so that it's a super slow drain anyhow. Then open and close the actuated valve every six hours. There ya go, I gave you an idea just to show I'm not completely raining on your dream
