Hello Sam,
There is an audio port on the front of the UC560. What my clients have done is develop their MOH announcements with associated background music and upload it to an iPod or MP3 player. We plug the audio cable from the iPod into the MoH port and configure the MoH port via CCA 3.2(3). Select the Configuration - Telephony - Voice Features-Music on Hold and select to enable the external MoH port.
As your client wants to add or change the announcements, they just need to play the track and loop it.
Hope this helps.
Bert