This product may meet your requirements. The "gold" version has a remote that allows you to select a trickle current that is not enough to start the car or disengage the battery, completely. I mounted mine inline under my car (not on the battery) making it completely invisible and unreachable without jacking up the car.
http://www.batterybrain.com/batterybrain.pl
It has the added benefit of keeping my battery from running completely down when I forget to turn off my lights.
Just an FYI.