i had a thought about something that might work which i'll be trying out in a few months. basically it's like the thing in the attached link and is designed for use on normal backpacks - i've still got one from when i was traipsing around europe 10 years ago.

http://www.corporatetravelsafety.com...roducts_id=108

i kinda figured that with something like that you could "wrap" it around the two rear panniers and thread a cable lock through it then d lock the whole shebang to something secure. weight shouldn't be too much of an issue - the one in the link says it's 1.5 pounds, which is like 500gms in the sensible measure but i reckon mine's a bit heavier.

folds down to about the size of two cigarette packets sat side-by-side. check the picture on the green bag to see how large the "opneing" is which sould allow you to get it around your two bags pretty comfortably before tightening it all in around the downtube and through some of the spokes on your rear wheel.