I just went and looked in my closet and one of the trenches I have is 100% polyester shell - it's the Gallery hooded trench from Nordstrom - I bought it during last summer's NAS. I think it might be the same one that MaryK has, but hers is brown (wish I'd bought the brown)...
You know what? It drapes beautifully. Very, very comfortable. Looks more dressed up than my other trench which is a Michael Kors slightly military-ish type of thing - that one is 97% cotton and 3% spandex. The MK looks more stylish but that's only because of the particular style - and the thing is? It's turned out to be an absolute lint magnet, and it wrinkles up very easily.
This is *not* the case with the Gallery hooded (100% polyester) trench. It does not feel or wear "icky" in any way. It looks really beautiful - like it's better quality than the MK cotton one, which is surprising - but true.
I would definitely try the coat you're thinking of - you might be pleasantly surprised. It's not going to be a 'stiff' material, but it will drape beautifully and hang well. It'll also be a lot more comfortable than you're thinking, and it works well in inclement (rainy) weather.
