Before you begin: you will need the English install discs for Arcanum, English patch 188.8.131.52, and the unofficial Arcanum patch found here (and the accompanying extra patch, optionally):
It is possible that this method will work with localized versions, but I don't think the unofficial patch currently supports them, and I don't know how well things will work without the unofficial patch.
1. In Crossover Games, go to Configure > Install Software, choose Install Unsupported Software, then New Bottle. Insert Arcanum disc 1 and use it for the install disc. The install will erroneously claim to have completed successfully and show you a game menu screen, but you'll need to quit this game menu and then click "Finished" on the unsupported software installer window.
2. Find the Arcanum install folder in ~/Library/Application Support/CrossOver/Bottles/[your new bottle name]/Sierra, make an alias of it for your desktop for convenience, insert Arcanum disc 2, and drag the files:
from the CD into the Arcanum folder.
3. Go to Configure > Install Software, choose Unsupported Install, choose to install into your existing bottle that contains Arcanum (ignore the warning about this being likely to produce errors), click on "Choose Installer" and browse to your patch 184.108.40.206 executable. It should install without issue.
4. Do the exact same process, but choose the unofficial patch installer this time. The unofficial patch installer will not actively confirm completion, it will just finish.
5. If you want the extra pack which goes with the unofficial patch (it's optional to this process), install it in the same way.
6. In Crossover Games, under the Programs menu, you should now see Sierra > Arcanum folders, which contains a few program aliases. Choose "Arcanum Safe Mode" to play Arcanum.
Remaining issues: none I've seen yet. Mouse movement is natural and disk load times are fast. There are sometimes briefly artifacts on the screen when the main menu loads, and there is a brief moment of blackness when a saved game is first loaded.
Updated after getting about 75% through the game: the game will occasionally crash when scrolling through a very large area, with a frequency of approximately once every couple hours of play. Otherwise, the experience is the same is a Windows install.