Thank you very much for all your answer, I'm glad to see this matter is of interest and that it does not seem to be straightforward (though, it would have been convenient).
This is not exactly a weka-oriented question (it would have been if there were a straightforward solution) though I'll seek for an answer, or at least some clue on the mailing list.
I'll dig the question with all I gathered. Maybe it's very simple using brute force or hacked algorithm, maybe it can be done with a formal, theoretical-based modification to Decision Tree Learning algorithm. I'll keep the list in touch of what I get.
Thank you very much,
-- Emmanuel Prochasson