Wisconsin Legislature Data
(b) Except as provided in sub. (4), no person may place, possess or transport a firearm, bow or crossbow in or on a vehicle, unless the firearm is unloaded and encased or unless the bow or crossbow is unstrung or is enclosed in a carrying case.
Other statutes: WI Department of Justice - Law Enforcement Services
Here's a OC Pamplhet for WI that I carry with me: http://opencarry.mywowbb.com/attachment.php?id=7901
On your bike, you would have to have it in a case, unloaded, and place the case in one of the saddle bags. That seems to be the best you can do.
Then should you decide to open carry in WI, make sure that when you holster up, you never set your gun on your bike, or in/on any other vehicle.
OC is not illegal, and there is no law saying it is legal. There is just a law saying concealed carry is illegal, and then the 5 places you cannot carry:
-No carrying of a firearm within 1000 ft of a school zone (unless on private property)
-No carrying of a handgun in a place that serves alcohol for consumption
-No carrying of a firearm in a state park or fish hatchery
-No carrying a firearm in a federal building
-Firearms in vehicles must be unloaded and encased, and should be in the farthest out of reach place in the vehicle
You shouldn't be harassed for carrying, but it's possible somebody might call the police to report a man with a gun. Hopefully the only thing that will happen then is that they police show up and question you for a short period of time. Be prepared for people to ask you why you have a gun. Having a pamphlet handy to give them would help.