Now this would be an excellent feature, this pbx is amazing, I've tested a few, and have to say your free version in my opinion is better than most of the commercial ones, in the sense of set up and features.
Now if you add this feature, I'd love it, it would rock.
If you do consider charging for this, is there any chance it could be a bolt on separate to your other licenses, to be competitive with NCH IMS which sells @ $69.50 for the retail version, I ask as I don't yet require your premium versions and this feature wouldn't be enough to entice me to upgrade, but if it was a reasonably priced bolt on it would.
http://www.nch.com.au/ims/index.html
I'd especially love it to connect with a shoutcast server along with another feature like:
+ Message scheduling to commence and expire on certain dates, times and to play only on certain days of the week or times of day.