Legal Matters in CA are going to be the biggest problem, followed by people struck with the "oh my gosh" -syndrome.
Scabbards for Bikes are nothing new, and are available in leather, including "leg of mutton" style to cotton duck to man-make materials.
These scabbards have been used in War Time , Harley Davidson's shown in past wars show these, as do bike [and bicycles] of the enemies around the world.
Here in the South, like other areas of the Country, Scabbards are part of 4 wheelers, tractors , combines and such. Not long ago, heading down a two lane blacktop, small rural area, an Old Norton, with a leg of mutton scabbard passed me - did my heart good to see that.
Kids still sling a rifle, and take off on bicycles to head down the road to shoot on someone's property. Kids, with a slung rifle / in or out of a case, and fishing gear, to do what kids do in the summer time. Shoot in safe areas with berms, and fish the farm ponds.
This is not CA though.
Political attitude is different here. Most folks carry concealed, no gun locks on the gun behind the kitchen door, over the mantle, or on that lever action or AR15 in the truck - in a gun rack, handy, when they come to town.
Guns are in the farm truck, tractors, combines , barns and sheds. Kids know the 4 rules and know how to shoot. Not uncommon to hand a kid a box of .22 ammo in broad daylight, be it in the town square, or the local diner.
Politics are going to be the biggest hurdle in CA.