I wanted to look into this one as well. Now my issue with the open carry attached to the side of the bike is this. Once you stop you will not want to leave the rifle opening attached to the bike and leave it so you would have to pick it up, put it away or touch it in some manner and could be an issue.
Most people who open carry a rifle on a motorcycle usually carry it slung across their back. It is possible for someone to obtain a rifle scabbard like the kind used on horseback, but that would be unusual. Open Carry of a rifle would usually only be done when going to or coming from a firing range, not that it wouldn't be legal just to take a ride to the corner store for a pack of smokes with an unloadedshotgun across your back. I don't see how taking the rifle out of a scabbard and slinging it across your back could be construed as "brandishing" as long as you are not doing it in a rude or threatening manner.
I can see how it should be legal to oc in a holster on your bike but what if you have ANY patches and a couple tattoos. Not saying anything wrong with that as we all have them BUT couldn't a cop try and push you into the mc group and try and get you charged with a gang related issue.I know this is a broad idea BUT I know many with rocker patches and YES they are simply mc clubs BUT try and tell the police that.
Many people are under the impression that if you are a member of a "gang", you are automatically chargable for carrying a firearm. The CA Penal Code does not address being a member of a gang. It addresses being involved in "Street Gang Crimes" and these are specifically identified in another section of the Penal Code. Unless you are actually involved in one of these crimes, there is no legal problem with wearing "colors". Of course this doesn't mean that the police won't try to find something to arrest you for.