You can save the mask from Bryce then bring it into Photoshop or some other graphic program like photoshop, and make the mask an alpha channel, and save as a TIFF file. You then can import the texture back into Bryce, and it will automaticlaly add the saved alpha channel. Or you could just add the mask into the transparencey slot of the texture, and save as a mat preset and re-use it over and over.