Doing it like that would massively simplify the plugin code, and allow the two plugins to be completely de-coupled, and therefore usable with other plugins rather than only with each other.

If foo-bar definitely needs a certain specific input then it can just do some sort of validation check that it's input is acceptable before it runs, and return an error message if it isn't.

aXML doesn't work the way you think it does (or want it to), it's different and its the difference that makes it special. Understanding how to work with it and leverage it's properties requires you change the way you think. If you can't do that then that's not my fault or problem, and you can happily ignore it and carry on as you are, again not my problem, and not aXML's problem.