Using Google Analytics to track the same session in client javascript and server side tracking calls?

We are implementing a 3rd party payment system into our site (Barclays CPI). We want to use Google Analytics to track where paying customers came from eg; keywords, email campaigns etc. However, the Barclays CPI returns the payment authorisation result via a server-to-server http request, not a normal web page with google analytics code. This breaks the analytics chain, we lose the client cookie that identifies the visitor.

Disable plugin / plugin action via theme

I have installed the Google Analyticator plugin on my site. I am also using the MobilePress plugin to serve up a theme designed for non-smart-phone mobile browsers. I have set up Google Analytics for Mobile for use in this theme and would like to disable the Analyticator plugin since it includes a call to a larger js file.