I don't believe so, but maybe since water is height based, any subtractive brush that goes below sea level should be filled with water, right?
updated: There is a checkbox to mirror every brush addition now. If it is checked building a wall will cheat in another wall on the other side of the planet. It's mirroring through the center of the planet. There is a new button to mirror all existing brushes as well. This can be used to basically mirror a complete map that was build on "half the planet". A few more brushes are available, the metal planet uses some composite brushes that I have split into single ones now Changed the way the server mod is created. It now copies the whole sandbox biome folder. So if you put custom files there they will be copied automatically. You should still make sure to create backups of your work in a safe place!