Sorry this took awhile. Here is the feedback from my colleague who implemented scripting changes to support the Virtual Hold product:
From the original design, calls will be sent to a CDN (unacquired) to the VHT application. If the wait is under threshold, the caller can opt to hold. If above, they can request a callback. For callers who hold, they are directed to a CDN which is acquired so the call is now under Symposum control. For a call back, we had a different CDN to go to the Symposium script. The call back CDN calls were queued to the skillset with a higher priority than the calls that were electing to hold so the call back callers would get connected to the agents first. That was pretty much the basis of the design.