The only problem I see, or better the only decision I would make, is where to place the temp and incoming dirs.
Groundbreaking news: files (or directories) starting with "." are considered "hidden" on *nix systems; user is not expected to look there. So, placing "Incoming" into ".aMule" is wrong and violates every HIG in existence.
So no news.
But, oh, I forgot ... aMule don't care about stinky HIG
So no decision to make. Let's make the changes to satisfy a standard we don't really care about. Let's change a running system, let's fix what's not broken.
But seriously, it can be done (technically), but would it change something? Where's almost in line with LSB, and we're almost in line with POSIX, xdg is maybe a benefit for apps that are designed from start, and maybe a benefit for apps that need or want to share their config with other apps for a reason I can't imagine. And IMHO we can benefit from some tools and technics xdg provides, and I still wait for 3.0 to start implementing some of these I think of they would be a benefit, but implementing this wouldn't change anything.
All apps that need to use the configs, are part of our source. All apps that want to use our configs, have to use them where we place them. Not talking about cas or plasmamule (maybe fileinfo, too), that don't use the headers that implement the filepath saerching and would need to be adjusted additionally.