I think you'd need to parse the random token from the response and create a persistence record then. Else, just by just looking at the request, the client would be load balanced again and then on that response LTM would add a persistence record. If the random token is only found in the response headers you could use HTTP::header $header_name to parse it and then add a persistence record with the persist add command. Else, if the token is in the response payload, you could use a stream profile and iRule like this:
Persist client on response content with stream
http://devcentral.f5.com/wiki/default.aspx/iRules/Persist_client_on_response_content_with_stream
A much simpler option would be to persist all requests using cookie insert persistence. This could be done without an iRule.
Aaron