@Eli - I'll give that one a go, although judging by the euthor's description it's not exactly what I'm looking for.
@Ego Eram Reputo - Well, obviously. But I was looking for something that could be a part of the creative process, not just a post-factum effect.
Thank you all! I guess I'll just have to keep using Krita for whatever requires this kind of mirrored drawing. Man, I hate its UI...