asterisk users - Dec 2005 - Terminating calls externally via SER

I'm wondering if anyone has ever implemented a scenario where calls
aren't terminated directly via Asterisk, but instead are passed back to a
proxy, such as SER to terminate the calls. With basic dialling, it would be
easy. For basic calling...
 
exten => XXXXXXX, 1, Dial(SIP/${EXTEN}@proxy,20,tr)
 
What about more advanced stuff tho? With Queues, the AgentCallbacklogin command
requires as an arguement where to reach the extension AND you have to supply the
context. Would Asterisk take something like:
 
exten => 8000,1,AgentCallBackLogin(${EXTEN}@context@proxy)
 
Don't know if Asterisk would correctly parse two @ symbols. Also, what about
hints? Would something like this be needed?
 
5551212 => hint,1,SIP/5551212@proxy
 
Would Asterisk even accept that? And so on. I guess I need to know if all the
Asterisk commands that require an extension can have a @proxy prefixed to the
end, and still work (assuming 'proxy' is defined in sip.conf of course).
 
Thanks.
Doug