interesting idea for sure. You might test it on a cheap lens, I would recommend something like Russian Helios 44, and to test it I would simply spray a shower on it than I would leave it for a month or so in a wet dark place. If the protection wasn’t good, you will most certainly find some rust on the barrel and focusing can get stiff or even blocked. Be sure however to screw any type of protective filter (I assume you don’t want to spray coating over glass?).
There is one problem to consider – probably most critical places for moisture to get into the lens or camera is the mount itself. Weather protected lenses have sealing ring , usually made of rubber, on the mount side. I am afraid that there is no proper way to apply nano coating in order to protect this weak spot, unless you spray it over with lens mounted on camera, but that’s a risk that I won’t suggest (and you won’t be able to change lenses).
In any case, if you dare to try it, please let us know how it went.