As a KTM rider I have no ignition key so I rarely leave the bike out of sight..but it only takes a second to jump on push the magic button and it's gone .I could wire in a hidden switch but that would be something else to go wrong ,so on fuel stops get the bike into top gear before it stops ,switch off fuel tap,fill up go inside to buy pastie and mars bar if scumbag jumps on bike the few seconds longer it takes to get bike off the line might be enough to hit perp with Arai or if he gets away chase until fuel runs out..use Arai (other makes of cosh are available).I could use the steering lock that KTM have thoughtfully fitted but an embarrassing incident involving such a device many years ago (outside college in front of the girls) on my Suzuki 125 -Lot's of rev's first gear drop clutch...you get the picture has put me off using it for ever.
Not going to help if they turn up with van and big KTM would probably pull away in top gear anyway, but every little helps.
I like your thinking, and you could even turn the fuel tap off before you even get into the station, so they don't see you do it as you dismount.