Rossco--
That's an interesting concept, esp. if you use a maintenance-free gel-type battery that could be laid upon its side, etc. and allows for shorter cable runs.
The parcel shelf is a relatively useless space whereas the areas on either side of the propellor shaft where the 6-volt batteries sat can make for good storage. I, like most BN1-2 owners, have gone over to a single 12--I think Group 27--battery mounted on the right side. I have a Tupperware-type container that fits into the left-side space, good for carrying spares that do not need quick or frequent access.
Can you post a pic? I would like to see the installation.