Start here for the actual text of the law & OC FAQ's. Our "open carry law" is 9.41.270, makes it unlawful to display a weapon "in a manner, under circumstances, and at a time and place that either manifests an attempt to intimidate or warrants (not causes) alarm for the safety of others."
OC on a motorcycle is no different than OC on foot, except that you "probably" need a valid CPL since it's a vehicle. Long as the gun stays in the holster, there is nothing to warrant alarm. You go waving a gun around, and you're going to get in trouble whether you're on a bike or not.
So in short, no it is not unlawful, as several of us have done so before. Every year 'bout this time there's talk of an OC motorcycle ride but it never goes anywhere.
Geeze and I thought I was the only poor sclhub awake at this ungodly hour.