I was seriously considering to make a browser plugin which would scrape pages for Google Analytics unique IDs, make a local database of them and randomly mix them up. Like this:
0. [page visited]
1. Does the page contain GA tracking ID?
no: do nothing
yes: add it to the database and replace it with another randomly chosen ID from the database
I'm by no means an expert, but usually when a web browser makes a request for a resource it includes a referral header which tells the server where it came from. This header is probably sent both to the initial javascript request and to the tracking pixels it requests to send new info to Google. At a first glance I feel like if you change the tracking id, and the referrals, you may have some success messing with their system.
But again, not an expert and I could easily be missing something crucial.