To answer your specific question, yes, native speakers would understand your meaning regardless of adjective order. There is a standard adjective order in English, as you're aware, but I'd be willing to bet that the vast majority of native speakers have never thought about it, aren't consciously aware of it, and are perfectly capable of understanding a sentence or phrase that doesn't follow the standard order. In fact, some poets have used different adjective orders for interesting effects (sorry, I'm at work and don't have any examples readily at hand).
At worst, it might take someone a couple of extra seconds to parse your sentence, to consider whether (as Peter Schor suggested) a long coat is a specific type of garment that's being described as lovely and white. Some people might take a moment to mentally rearrange the words to the more common pattern. If I heard the phrase "long white lovely coat", I might briefly wonder if I'd misheard, and the speaker was actually speaking of a long white overcoat. Still, assuming I heard the words correctly, it's very unlikely that I'd fail to understand the meaning.