yes, i have. A couple of times i've done the walk home of shame.
I carry around a RC lipo battery at 'storage charge' in the glove box and a small 10A current limited DC/DC converter
The capacity isn't a lot with it at 40% storage charge however i don't need much to fully recharge the supercaps.
Leaving the lipo in the car at storage charge is much better for its lifespan.
The only real risky bit is going for a vehicle cert/warrant of fitness.
As I don't know how long they might leave the car with ignition on but engine off. Or leave the lights on.