I think PDS is asking if the throttling could be done before TMM translates the client IP to its own source IP. If that's the question, yes, you can throttle based on the original client IP. If you're throttling based on layer 4 connections you can use [IP::client_addr] to get the client IP in the CLIENT_ACCEPTED event. For HTTP request based throttling, you can throttle in the HTTP_REQUEST event.
http://devcentral.f5.com/wiki/iRules.client_accepted.ashx
http://devcentral.f5.com/wiki/iRules.http_request.ashx
http://devcentral.f5.com/wiki/iRules.ip__client_addr.ashx
Aaron