I was able to get the game to run with Crossover 7.2 (OS X 10.5.6) by using the "Configure Divine Divinity" utility to set the graphics to DirectDraw. I then installed the November 2007 distro of DirectX using Crossover's installation system. After that, the game ran just fine without having to manually set the system to thousands of colors.
Unfortunately, I cannot get this to work for Beyond Divinity as there is no way to change the renderer to DirectDraw in that game.