I would think you can put your customization in the customization point at the top of OnHoldAudioRandomizationMonitor called CustomOnHoldAudioRandomizationMonitor. There will be a call attribute called Eic_CallDirection which can have a value of I = Inbound, O = outbound, or "" meaning manual dialing state. To prevent something from breaking I would look at recording a 5 minute wav file of silence to be played. I don't think you will run into "" in your customization but I would double check. A simple check of the attribute and if it equals O to set the attribute Eic_OnHoldAudioFile to the silent wav file. Now any outbound calls will attempt to play the silent wav file instead of the built in hold music.
*DISCLAIMER* I take no responsibility for what you do with my suggestion above and if it breaks something. I have not tested this but have simply gone off memory of how this part of the system works. Always test your changes in a nonprod environment before promoting to prod.