Exactly, you're right. Acer palmatum, amoenum, and a few others are among the most mutagenic species. What's more, some can interbreed, so there can be hybrids between some species of maples, and even between cultivars.
Sometimes, they are from "witches brooms", in which case they are propagated by grafting.
It's possible, and even somewhat likely, but young seedlings need to develop for a few years before really showing their characteristics, and then one has to find one that is really original - there are alreday about 1,000 of Japanese maples registered, maybe theree times that number if you take all the synonyms or the "like".
After that, it needs to be propagated (grafting) to make sure it can keep its characteristics, and live more than a couple of years.
Then, it has to be registered.
And yes, you can always find a specimen that looks different, interesting, that you like. I have a couple that I will never "register", some would perhaps discard them, but I like them. Here are a few that make me wonder, and I think they'll disappear when I'm gone. Just like the people I met, I liked being with them, now...
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Huh huh, no, I'm not in that kind of business