Do I take it this never got fixed? I've come across this thread while looking for a solution to problems getting Pillars 1 running on M1 Mac, but if there are other problems with the Mac version anyway, maybe I should just bite the bullet and use Parallels to play the Windows version.

Having no visible cloaks/capes (you can equip them and nmechanically they work just fine - just no visible model) is not a bug. Obsidian chose to use a commercial Unity3D-plugin for the cloak/cape physics - unfortunately that plugin only works for Windows but not Linux or MacOS. Thus it can't get fixed by Obsidian (otehr than discarding the plugin and developing cloak physics themselves which was not feasible back then). 

So yeah - no cloaks for MacOS (or Linux).

I always played the native Linux version - maybe I could see cloaks if I tried the Proton version from Steam (I doubt it but maybe...)

In Deadfire this problem doesn't exist aymore because Obsidian chose to use their own cape physics etc. right from the start which works with all platforms.