I think I just answered my own question, seems like I need to be extracting the contents of /path-to-play-services-project/libs/google-play-services.jar which actually contains lots of stuff, and corrects that link error when I update my extension jar file with its contents. The following seems to sort it:
jar xf /path-to-play-services-project/libs/google-play-services.jar
jar uf ./my_gplus_extension.jar ./com