Hey all,
So a while ago I tried using the
/ui layout [save/load] [filename]
command while on the Mac client to preserve my UI when working my way through completionist. I didn't get it to work, so I wrote it off. Today, I had the urge to see if I could figure out why it didn't work, and more importantly to see if I could get it to work; I did.

Unlike the Windows client, which saves roughly to Dungeons & Dragons Online -> data -> ui -> layouts [1], the Mac client doesn't actually seem to have a default location to save/load from. What this means, is that 'filename' in the command is instead 'filepath.' So I actually get to choose where I want to save and load from.

Step by Step:
open your Documents folder and create a folder for DDO
in the game, use the command:
/ui layout save /Users/[yourUserNameHere]/Documents/DDO/[filename]
So if I wanted to save a file named 'archer', I'd use the command:
/ui layout save /Users/oberon131313/Documents/DDO/archer
Loading the file works the same way:
/ui layout load /Users/oberon131313/Documents/DDO/archer
And viola! UI saving and loading on the Mac. Enjoy!

(Advanced note, filepath doesn't actually matter, as long as the game has access to read/write to the directory. ~/ to access user dir doesn't seem to work.)