I would say that there might not be people who have tried SIP integration with Asterisk on here, as most of the focus is on 3300's and 5000's with the handsets running MITAI rather than SIP.
There might be someone that has tried this, but it might take a few days for people to log back on to the board as I'm guessing that not everyone looks every day, so you might yet get a solution or suggestion.
My only other suggestion would be to have a look at tek tips and see if anyone has any experience with this on there as well as there are some people that only post on there and not here and vice versa.