Seat Switch is required by Outdoor Power Equipment Safety standards that cover a LOT of countries. Don't have a choice, from the smallest garden tractor to ANY tractor than can be fallen off of or turned over. That standard pretty much includes ALL tractors and a lot of other equipment too. If the safety being completely mechanical is a need, I guess you could tie a string to the coil wire if you had a gasser. But since there's no ignition system on a diesel, that's a little harder to do with a string, but I guess you could still come up with a butterfly in the intake that would shut off air flow.
The seat switch is to shut the engine down if the operator leaves the seat and the transmission is engaged, or the PTO. Kubota has been generous enough to provide a new option on the LX models that allows operating the PTO with the seat still down by locking the brake, putting the range selector in N, and then pushing an override button for about 10 seconds. This allows using stationary equipment like chippers. Miss one detail, you shut the engine down. Simple safety. It works.
But back to the subject at hand, I'm not quite sure that a chain that heavy would be required, and I'd probably opt for some kind of pin latch if I was that worried about it coming out. If 'sticks' come up behind that adapter plate, stiff enough to operate those levers, you're about to either rip a front tire apart or relocate your radiator to the 3 point hitch. As someone else pointed out, they latch by cam action over center and are spring loaded. On top of that, you didn't pin the actual latches, only the handles. You did know those handles can be fully engaged and the latch still not fully engaged, right? What's to stop the same 'stick' from pushing the latch pin out of it's hole? The latch pins not fully engaging is more often than not what creates hazards, despite the handles being fully down. There's no substitute for a visual confirmation that the latch pins are fully engaged. So, chaining the operating handles really isn't going to help much to prevent the problem you're targeting.
To each his own, though. Not criticizing, just debating.
