I like this approach for role-playing an aversion to metal armor. The druid I'm playing right now is wearing metal scale mail but I expect for him to be called out on it the first time he runs into another group of druids. Then he'll probably feel kind of shamed into finding an alternative.Personally, I would have no problems with allowing a druid to wear metal armor. That said, followers of the old ways would be likely to look down on the druid for doing so, since the druid is breaking with tradition. Druids in my campaigns are often held in respect by creatures of nature (treants, dryads, etc) but they'd be inclined to believe that someone wearing metal armor is trying to (poorly) impersonate a druid, and therefore would be quite dubious of such an individual.
That said, it's entirely feasible in my campaign to obtain versions of metal armor that aren't made of metal, so it's not something that's likely to come up. I suppose a druid could also obtain glamoured metal armor, but that would likely be more work than simply acquiring armor made from alternative materials.
It's kind of weak, character and story-wise, but absent a hard rule, a more compelling lore element, or a specific penalizing game mechanic, it's the best I've been able to come up with.