As the initiator of this thread, and encouraged by other posts, I've been following this up quite vigourously.
I looked at batteries such as Kim has illustrated above, but found them quite bulky and heavy - several pounds at least. The cost here was comparable to Kim's item at around $30 without the charger. I now intend, after a bit of experimenting, to use the 12V rechargeable battery from one of my cordless electric drills. It's very robust and carries a good charge for its intended heavy-duty drilling use, so it should be ideal for driving the pump. It is also very light at only 400 grams, or < one (1) pound in your US money. I intend to remove the battery from the craft after every outing so as to be able to still use the drill and to ensure it's always charged - although I hope that pumping activity will be minimal!
Proper waterproof cases as used by photographers are quite expensive at about $35 for the size needed ('Pelican' brand), so I'll mount the battery inside 2 x good quality waterproof 'Tupperware' type plastic containers, one inside the other for optimal waterproofing. Two full cycles in the dishwasher have proved their watertight qualities quite adequately!
The battery will be well padded with stiff foam inside its container, and that one well padded inside the larger outer container. This will then be strongly secured by a nylon strap on the hull bottom in an epoxied-in ply 'frame' immediately behind the bulkhead so as to keep wiring to the switch and pump to the shortest length possible.
The pump (about $25, 'Johnson' brand, made in US, pumps 450 US gallons/ hour) weighs only a few ounces, and has the minor benefit of a built-in bracket which will allow it to be screwed onto the aft cockpit bulkhead behind the seat, nice and secure. It's very compact, too - only 4" tall and 4-1/2" diameter at the base, about the size of a small cup or mug. An alternative 'Rule' brand item, also US-made, is even smaller at 3-1/2" x 2-1/4", and a 500 gph unit costs about $38 here.
A waterproof switch is very cheap at about $2.40 from our Oz equivalent of Radio Shack or Tandy, but if necessary I'll waterproof it further by putting a rubber finger-stall over it, nice and tight. An alternative would be an automatic float switch, at about $37 ('Johnson' brand again). All up, with PVC hose, through-deck hose outlet, switch, wiring, I reckon on no more than two (2) pounds of added weight and a total cost of under $40 using the cheaper options above.
For me, this will bring the feeling of a little added security 'out there' and the joys of fiddling and doing something a bit different! I'm not going to retro-fit this to the Ches 17LT, but it will be built-in to the new Shearwater 17 when construction starts soon.
An interesting challenge - thanks to all for input and ideas...keep on reprting.
Cheers...
Wordsmith