ive done it, use a nice proper padded comfortable rucksacke with a waist strap and a long cord with an easily detachable connector.
heres the connector
heres the bag and bike
i had a 12v 26ah sla in the bag. i couldnt really feel the weight. the nice padded hikeing bag with the waist strap definately helps. a lipo would weigh nothing.
pros
there the stealth advantage
easy to swap batteries.
easy to charge batteries (if you cant take you bike next to a plug but you can a bag)
expensive batteries arnt left on bike in public to rob
any extra battery weight is under your "control" and not the bikes
you can get quite big bags for quite big batteries.
cons
if you fall off (i have) its pants if the cable doesnt detach
getting off and on the bike is a faff and make you look silly as the cable gets in the way
the extra cable length could be robbing you of a few vital volts and amps.
slas are heavy, dont try sla unless your after exercise
theres probably more to it but thats from my experience