I haven't looked at the sixaxis setup in detail but from what you describe, i suspect that they are providing the same file in their package, so the package manager sees that another package would collide and refuses to let you install it. All it requires is for the sixaxis maintainer to either depend on the joydev package instead of installing the module, or to mark the package as "Provides" joydev. If his package advertises itself as providing the virtual package "joydev", the accelemymote package that depends on "joydev" will happily install and use that module. The first option is better because there are obviously cases where someone would want to uninstall the sixaxis package and not the accelemymote package, and that wouldn't be possible with the latter option.

Maybe you can point him to this post so he's aware of the problem?

Edit: of course individual users can "force" the install, which should work fine, but that's not a solution.