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...no, I have never seen this done successfully.

What you might try is to carefully mask off the original decals when you do the color coat, then unmask them and cover the whole thing using one of the urethane clear coat products, which go on pretty thickly, in order to flow and achieve a high gloss finish. I have pretty much gotten over the whole "decals have an uneven line from the paint" business, because most of what I repaint is done with vinyl reproductions. I always have a line, and it doesn't bother me under the clear coat urethane.