You've asked some great questions.
First of all, Fluid Film will make an excellent vehicle undercoating. It is a once a year type application that will stop any corrosion from moisture or road salt. Because it doesn't dry, it will migrate into inaccessible areas and resist wash off. We sell a lot of Fluid Film on the East Coast and Canada for that application due to the harsh winters.
I have never heard of the plow mold board application and would appreciate any feedback you could give me if you try it, but if what you are looking for is a slick surface to keep dirt and debris from sticking it should hold up fine.
Under the deck of a mower Fluid Film will stop any corrosion from occurring and really help keep grass from accumulating. When I am done mowing, I usually just hose out the undercarriage and put a good coat on. I know firsthand how well it works.
Fluid Film applied over existing corrosion will stop it from spreading further on contact. It will loosen up rust scale and make it easier to clean.
As far as spraying Fluid Film directly on plants, a few months ago I sprayed Fluid Film directly on a bush in back of our plant to see what kind of an effect if would have, and it had none whatsoever. We have not, however, had any laboratory tests done or tested its effects on different plants, so I am hesitant to recommend its use for this application.
I hope I've answered your questions to your satisfaction. If you have any more questions or comments, please don't hesitate to ask.