XPC services must be located within the host app's bundle [1]. They must be appropriately code-signed. How do you propose to use this mechanism for 3rd party plugins?
As I've pointed out elsewhere in these comments, XPC isn't required for supporting plugins in a sandboxed application, and it's possible for plugins to exist within a signed and sandboxed app's bundle:
[1] http://developer.apple.com/library/mac/#documentation/MacOSX...