Doesn't always work that way. It's floating under the surface. You can't see it, until you hit it. I have done something like that twice with daggerboards. The first time was a piling about floating about 6 inchs under the surface. The second was the "stub" from a broken channel marker (think telephone pole broken off 1 ft under water). In both cases it resulted in a violent pitchpole.

Going back to the original question. A centerboard that will fit in the daggerboard trunk will be too small. Fitting proper sized centerboards will involve cutting up the hulls. You could try fitting leeboards. Leeboards are basically centerboards that are on the outside of the hulls. If you put them on the inside of the hulls they are not quite as ugly. Because flow is not bounded by the hull they are not as efficient as centerboards. To make them work, you will need very good foils.