There appears to be a problem with the stub mechanism in XINS. Adding a <impl name="implname" /> to an existing project, and running the stub-<api>-<alternative impl name> command ends with the complaint that <alternative impl name>/impl.xml doesn't exist. After manual creation of this file, all seems to work, until you deploy the war. At runtime, the implementation breaks when the original imp.xml has a default custom calling convention dependency.
Adding the dependency to the stub impl.xml file and rebuilding the package solves the problem. BTW, I didn't test this with a new API, but I expect the same behaviour.