FYI, we are doing this also. In our over-flow message we just found out you could add in audiotex recording to the day and night greetings of the CRA, to announce the callers position in Queue, and expected hold time.
Audiotex Recordings
Q = Queue position
T = Expected Hold time
The hold time is calculated off the Hunt Groups Timer "avg call length" I think

Our customers have been happier just knowing how long they have to wait