Can you just unhook the safety switch on the seat? I think pulling the wire will keep the engine from cranking. It's not an obvious thing. Faulty switches have caused many a machine to visit the dealer.
Also, leaving the PTO lever in the engaged position will bring that safety switch into play.
on edit - Just thought that if you can rig a pad lock so it sits under the HST peddle and is large enough to raise the peddle up a bit. That will trigger the HST interlock. That should also keep it from cranking. Additionally, getting a quick disconnect steering wheel tends to confuse the inept crook. You have to take the wheel with you ...
If you have been broken into in the past, have you thought about getting an IR game camera? No flash, no sound, high quality 4mb pictures. You can make a disguise box and have it aimed to where a person on the tractor would be photo'ed. The cost is down around 200 bucks now.
Must sux being in a place where you are in fear of theft. Around here, the neighbors know what cars are supposed to be where at what time. At first it was disconcerting to have someone ask who was at your place with the red truck. Around here, it's not the mountain lions or even the wimpy barkless dogs you have to worry about....
jb