A lot of people use 100LL (illegally) for off-road vehicles. If it isn't fuel injected, and doesn't have a catalytic converter, it generally will work. The potential fine is definitely not worth it though. When I worked as a line-man in college at the local airport, we would occasionally get folks who would come over to the self-serve station, pump 100LL into little gas cans, and then leave. More than likely this is what they were doing.....
You can definitely run Jet-A or equivalent in diesel motors. I have also heard that the PT-6 can run off diesel in undeveloped areas (ie on a short term basis) where no JP or Jet fuel is available....anyone else know anything about this?