First off, it sounds like a cool idea!
You may want to check out the marine plants/macro algae forum. The sticky, "Forum favorites (old helpful posts)" is a good place to start. Samala is the expert on seagrasses. Also, my thread, "Caribbean Biotope Seagrass Tank" has lots of good info in it.
I'd suggest researching the various seagrasses available and deciding which one(s) you want to keep. This will inform your decisions on tank height, sandbed depth, current, etc.
The tricky part will be the balancing act between keeping corals and grasses happy. Corals need low nutrients, plants like high nutrients. However, seagrasses are adapted to low nutrient conditions, so they are a better choice for a reef tank, than say, a nutrient-greedy caulerpa.
I'm just scratching the surface here, but I hope this helps. I'm happy to answer any questions you have. Good luck!