One time when I put my tank back on after removing it to service something, I managed to put a kink in the rubber overflow tube that drains to the ground and had water collected in the that little overflow area.
I don't know all the details about how the evap canister works and all that it's made of, although I want to better understand it. From what I understand it's charcoal though and I'd be surprised if there is anything in there that would rust and send it back up to the fuel tank. That just sounds like poor design, and I'd like to believe that Kawaski has been around long enough to have engineered better designs, but who knows???
What I understand about the evap canister is that if it does get wet, it will dry out and should be fine (according to the customer service rep I spoke with at Kawasaki USA). There is also a fuel separator that somehow magically separates fuel from fumes (I'm guessing) and directs the fuel back to the tank (again, if I understand correctly). The fuel separator connects to the tank towards the front of the tank (The hose with the red dot). The metal tube in the tank that it connects to is inside the tank towards the front also. It's not the same as the overflow tube under the gas cap. I don't know how it all works, but I recently spent a good deal of time looking at it when the fuel line from my separator to the tank got a hole in it and I couldn't find an OEM replacement. I was considering other options (short of removing it). I want to fully understand how this whole thing works, as my separator seems to have gas running through it regularly (I'm still getting a little leak out of the aftermarket tube I used to replace the busted one).
Hopefully this is helpful. I know it doesn't really answer your question, but maybe it will help shed some light on the evap system and others have bits to add...
2011 Imperial Red
Givi V46 / V35's SW Motech mounts
Skidmarx hugger Moto Werks bar riser
Mondo Motors LED Driving Lights
A little bit of road rash