It isn't so much the cold which will strt the coat, rather the shortening daylight hours. You could use a cotton sheet (easier to keep clean) in the stable and maybe put a light on if your stables are quite dark. The best effect form a rug is keeping the coat 'flat' but you can end up with a fluffy neck!
You can't avoid a winter coat - it will grow reagardless - it depends on daylight hours! All a rug will do is to keep it flatter but it will still be there waiting to fluff up the minute you remove it!!!
Is there no connection to how cold a horse is and how thick the coat grows? I ask because PF was a real teddy in the UK (where obv it's colder in winter than here) but now we're here, she hardly grows a coat at all... although... *muses* I don't think the days are quite as short as in the uk ...
My woosy one has been having a LW turnout rug on probably about 70% this summer. Its on right now! But then to be honest, our weather up here isnt nice than often anyway!
I wouldnt worry about rugging to avoid a winter coat as you will get one anyway. Only rug if you feel your horse is getting a bit chilly.