Plastic clips and cloth flag would probably be fine - as long as there isn't any metal you should be ok as far as the RF function of the antenna. Metal attached to an antenna or in the antennas near field can couple with the antenna even with out being connected to it. For instance my HF antenna in the backyard couples to the verticle run of the AC wire in my house on 14mhz approximately 20 feet away - causes the CO detector plugged in at the top of the verticle run to spazz out - also causes grief with the resonant point of the antenna ever so slightly changing its radiation pattern and feed point imepdance.
Next the additional weight of the flag while itself isn't much - wind adds much more weight to it via the drag created. Now flexes in a different location than originally designed - shake your antenna hear that rattle - that is the element of your antenna rattling around inside the antenna - the feed point is only assembled with tin based solder - the pic below is of the feedpoint of a Shakespeare 5206 (this isn't a galaxy antenna and now you see why if it doesn't say galaxy on it don't buy it).
Another dog used used this antenna for a flag pole - than after a while wondered why his radio didn't work too well - feedpoint became damaged and the antenna no longer worked on the Marnin VHF band. While a galaxy might take more abuse since it is assembled from copper pipe it is still only soldered together with tin solder like a plumbing joint.
Call me overly cautious, but that antenna may make the difference someday - go ahead put it on your FM antenna at least if that one breaks you'll only miss the news or the music.
If you want an empty 5206 radom to use as a flag pole I got one in the storage area.
Chromaflage
Radar Reflectivity
Linear Mode
