The only anemone-proof barriers I've seen are 1)glass/acrylic, 2) screen (netting, craft mesh, etc). It needs very small holes, or it will not work completely.
I have personally seen large (8" expanded) BTA's squeeze through the 1/8" slits of an overflow box. Eggcrate really will do nothing to stop an anemone from going through if it decides to. Also, as others have already stated, flow will be an issue. I know you say you will have a powerhead on each side of the tank, but you are creating an area of restriction, which will lead to dead spots for flow in your tank. I understand your goals, but practically speaking I do not see it working as well as you may. I see it being a lot of time and effort for long-term frustration.
Additionally, even if it works on day 1, I do not see it working long-term, or looking good long term. If you have excess nutrients, the sand/epoxy-coated eggcrate will be a perfect spot for algae to grow. Good luck removing the algae without also messing up your sand/epoxy layer. If you do not have excess nutrient problems, the eggcrate will not stay sand-looking, but rather will grow coroline algae. In this case, I don't see a way that it won't just look like what you have described - sand/epoxy-coated eggcrate. It will not be natural looking, and IMHO will look just a bit awkward.
BTA's don't walk nearly as much as some people believe - IF you can provide for them an adequate environment. I don't see why you couldn't have your maxi's on one side of the tank, and the BTA's on the other. Seperate by islands, and start slow to make sure they do not walk (i.e. you are providing adequate environments).