I had a thought -- do you have any control over your BES? I maintain the caching proxy server at work and write the AutoConfig (PAC) file; if you can somehow get your BES to return 'DIRECT' mode (or whatever the BB equiv. is) back to a proxy config request from the device, you might be in business...maybe.

(I've never even seen a BES so run with the idea in the proper parlance...)