Has anybody ever relocated the battery to the rear of a [DSM]?

There are many videos on the relocation of the battery to the trunk. See Jafromobiles GSX battery Relocation Video. It is important to choose the right gauge of wire to use.

To choose an adequate wire gauge, determine the amp draw (amperage) that the wire circuit will carry.  Then measure the distance that the wire will travel (length) including the length of the return to ground (the ground wire running to the chassis or back to a ground block or battery.  Using these two numbers, Amps and length, locate the nearest gauge value in chart below.  For 6 volt automotive systems typically a wire gauge 2 sizes larger than what is shown should be used.

SRC: http://www.offroaders.com/tech/12-volt-wire-gauge-amps.htm

Other items you will need are a marine battery box (or fab your own), fusible links and distribution blocks), venting tube if not a sealed battery.

Since many proponents claim this reduces the understeer on DSMs, there has been quite a bit of discussion over the years on this subject.