One of the questions I didn't ask is what model bike are we talking about here. In my minds eye I trying to imagine where a flagpole would mount on an 1800 other than a trailer hitch. Besides the sub frame, it's all smooth Tupperware, except the license plate bracket...hmmmm.... the license plate bracket....hmmmmmm....Please report back your findings when you do find one.
Got to agree with Santini; on this one.I don't know where you would mount a large flag pole without ripping tupperware off the bike ! I wouldn't mount it anywhere but with a hitch !! Good luck !!