Martin asked: Is it illegal to attach a small (3’W x 6’H) mobile billboard to a bicycle to provide mobile advertising services to local businesses?
Your bicycle is a private vehicle and you place anything on it that isn’t prohibited by statutes, such as flashing blue lights. I am not aware of any such prohibitions on signs and assume they are allowed, just as they are on cars and trucks.
The only caution I might consider is that the business should be legal and registered and licensed if required by the state. That is beyond the scope of this site.