Hello @Michael, thanks a lot for your fast response and thanks for your suggestion.
Really my custom xml plugin has more busines logic (like project type recognition based on file path, decide which files are checked for each check, etc.) in addition to metrics, highlighting, etc. but after thinking it and especially after your response I think that “merge” both plugins in one is the only possible solution to maintain both checks and logic, so I will do it.
Answering your question I was happy with my custom plugin until I found this problem 
That is, I think that the SonarJava API is a bit restrictive if you need do things more complex that develop custom rules as in my case, so for me fork the plugins was a great solution.
Anyway I don’t think it’s a bad idea to have an API in order to develop a bit complex XML checks.
Out of curiosity, can I ask to you why you added these limitation to files highlight?
Thanks a lot!
Regards,
Irina