What is the game client included?
The actual client is the 9th anniversary one (patch 6.0.1.10), which you can change at any desired moment.
Does the UOPatcher works?
Yes, but you need to change a few things.
You'll need to open the MacUO package, then go to the "Contents" folder.
There, you'll open the file "Info.plist". This should open your text editor / code editor (here it opens Xcode).
At Info.plist, you'll search for the tag "Program Name and Path", which probably will be pointing at Razor and its path (/Program Files/Razor/Razor.exe).
You'll change it's value to /Ultima Online/UOPatch.exe or it's equivalent.
Then, you have to run the MacUO.app again.
Can't guarantee it will update it correctly, but that how it works.
After the update, you'll have to change the "Info.plist" tag again, to the Razor folder.
The most secure way, really is to get the desired Client version from somewhere else, and apply it inside the package, as stated below.
How to change the patch / UO version:
You'll only need to open the package (or in this case, MacUO.app) and change Ultima Online folder. You can either change only the content of the folder, or the entire folder.
IMPORTANT: The folder must be named "Ultima Online" and it must be placed at the "drive_c" folder.
How to modify Razor / UOAM:
Razor is inside the folder "Program Files/Razor", so everything should be easy to modify.