With the brake fluid res. cap off and the right tool to reseat the rear piston, I'm sure it can be done with someone holding the caliper for you while you twist. It might take some muscle, but I doubt nothing extraordinary.
I'm not exactly sure how the setup looks on a G20... maybe you can try unbolting the caliper completely, then maybe bolting the top of the caliper to the bottom of the caliper mount?
Just an idea
