Hi Rich,
Have you thought of approaching this a bit differently. Have the F5 more responsible in maintaining the connection based on calls per second or numbers of session - such that the SIP proxies wouldn't have to send a 480 error code.
There are some iRules that were already made for some kind of rate limiting connections
Here are some that you can possibly leverage (NOTE: Some may require v10.x)
http://devcentral.f5.com/wiki/default.aspx/iRules/virtual_server_connection_rate_limit_with_tables.html
http://devcentral.f5.com/wiki/default.aspx/iRules/LimitConnectionsFromClient.html
http://devcentral.f5.com/wiki/default.aspx/iRules/Version_9_x_Session_Limiting_iRule.html
You can also go to http://devcentral.f5.com/wiki/Default.aspx/iRules.HomePage which provides a list of all iRules that many in this forum have contributed.
I hope this helps
Bhattman