Others have moored their water ballast boats in tidal areas, but the caution about rocks and sand damaging the hull and paint are worth considering. With the centerboard fully retracted the boat should settle on the mud with no problem. You could have problems over time with crud jamming up the centerboard slot. Also you want to avoid anything that results in flexing of the hull - you could compromise the ballast tank seal.
All in all, I'd look for another solution. After all, this is a trailerable boat. I dry slipped my boat with the mast up for a couple of years and it was very easy to launch and retrieve. The boat stayed very clean and I saved a lot of money that way.