It gums up and makes everything a mess and you'll hate yourself for doing it. It burns really smoky and since the temperatures involved are way past the point where it will just vaporize and immediately catch fire it wont be around long enough to help lubricate the motor once hot.
Castor oil is just more resilient to being heated than other plant based oils are, that's why it's used like it is (or mostly was.) Almost all oils will combust if compressed and heated enough, it's a form of autoignition (when something gets hot enough to start burning without actually being lit on fire, which is what a spark plug normally does to gas.) You could also ignite gasoline, alcohols, and flammable gasses in the same way.
Certainly if it worked people would be doing it, vegetable oil is extremely cheap compared to 2 stroke oil, so there would be a big user base if it was feasible (as in not a bad idea.)
