Just wondering if it is possible to install the Ultima II and III fan patches/updates to the Mac version of the GOG Ultima 1+2+3 bundle?

I have a look through the Contents of the app files, but could not find any location in it where the DOS files for the game were.

Thanks

Contents -> Resources -> NameOfTheGame.boxer

GoG's Mac versions use a custom standalone Boxer, you will find the game's DOS files inside the gamebox. Personally I'd recommend taking the gamebox out of the app and playing it in plain Boxer, the you can use the nice Roland MT32 music from the fan patch.