Not an expert, by any means, but will make comment based on years of being around many of the repop sailing ships up and down the east coast.
Usually tying anything to a vertical post is done with a clove hitch or square knot. Both are easy to tie and untie. Not as easy as a bow but a bow will collapse as mentioned. To lash something to a vertical so it won't come loose, a marvelous knot is the constrictor. http://www.animatedknots.com/constrictor/index.php?Categ=boating&LogoImage=LogoGrog.jpg&Website=www.animatedknots.com Cinched down this knot will not come loose without some work and the help of a tool, such as a marlinspike.
I've got a repop Battle Flag in my office tied with a constrictor on top with 2 square knots on the other ties.

Just an opinion.

JB