Mounting the battery in the tail involves many feet of heavy copper wire. Much of this wire does nothing for the CG and decreases the performance of the electrical system. I haven't run the math but it would be interesting to see how the CG would end up if you took the difference in weight between the two systems and substituted a lead weight just forward of the stern post.

Alternately, with the battery mounted near the engine you could:

Expand on Skip's idea and substitute a tool / survival kit mounted far aft. Wouldn't this count as "useful load"?

Place the standby battery in the tail cone.

Use a light weight propeller (MT).

Mount the batteries just forward of the spar (assumes using RG batteries).

On my Lycoming I had the option of using a lightweight starter and alternators.

Remember that RG batteries can be mounted in ANY orientation. I tipped mine back 14 degrees and used the firewall as one face of the battery box.  The box itself was 3 BID glass and 2 BID carbon with phenolic hard points for the solenoids. It is covered with Zetex A600 for fire and heat protection. (picture attached). The wiring connected between the battery, master solenoid and ground power solenoid has fire sleeve for added protection. Fire sleeve was also used on the engine wiring harness (Lycoming starters and alternators are mounted forward).

Regards
Brent  Regan