Actually, Armory implemented message signing
before Bitcoin-Qt did. Thus, Armory had no way to know how Bitcoin_Qt was going to implement it, and thus we ended up with different, incompatible signing algorithms. So, while Armory has a "message signing" feature, it's not compatible with anyone else.
There's an
active bounty out to fix this. Hopefully, it will happen soon. But if you are compiling the list for inter-compatible signing, Armory is not one of them. This is also why Armory's signing is kind of intermixed with an ECDSA calculator... I started with that, then decided not to upgrade/polish it until I got a compatible algorithm implemented.