The upcoming CUBAC 8.6.1 release (FCS currently slated for June 2011) will add Wait Time as a queue overflow type, which could be used to achieve what you're looking to do.
Today, each CUBAC queue can be configured with an overflow destination (any phone number - could be an extension, could be another queue, could be a voicemail box), and there are two triggers to send a call to the overflow destination:
No Operators (checkbox): if checked, then calls will route to the overflow destination if no operators are logged into the queue.
Number of Calls: say it's set to 2. If two calls are waiting in the queue, any additional calls into that queue would route to the overflow destination.
In CUBAC 8.6.1, we'll be adding Wait Time overflow: if a call has been sitting in queue for x seconds waiting to be answered (x is configurable for each queue), send the call to the overflow destination.