Not a 100% solution obviously.
But the Swift Bandito bar/seat bag has slots where the buckles can reside inside the bag, rather than outside where someone will notice it can be easily detached.
My bandito survived at least one year and the better part of a second outside in the bike theft capital of the US, Brooklyn, NY.
Dunno much about the sackville seat bags, but maybe you could look at having holes cut to pass the straps thru and have the buckles inside?
Patrick's QR idea is probably the only truly secure method.
For me, contents are a concern tho. Didn't find a solution to making the contents pilfer proof, but my tool kit only got stolen once during that time.
The bandito bag has a pretty unnoticeable access to the pocket between the liner and the outer, and I kept stuff in there w/o fear of it being stolen apart from someone taking the bag as a whole.