Rob might have to answer that one...
Seems like the best results are coming from those softer/more supple cloth type. Once you get into the '70s, the stiffer "felt" (or whatever it is) is a whole different animal. Not sure how that material would react to the bleach.
I have passed on bidding/purchasing dozens of white pennants over the years, because of ugly staining. So am hoping to see some more good results!