Criminy! I can only recommend to leave while you can.
No, seriously, I got into writing such a
timeline, and I was overwhelmed by how much research I had to do to design anything hardly reasonable. As you seem to be working on the same topic, I would of course very much appreciate your comments on my TL so far.
Needless to say, I would enjoy to follow your tl when started.
As to your question:
First, I cannot imagine that a peace treaty would question the
nominal existence of any independant state. OTL's 30-years war caused a maximum of destruction and
revolution, but the Peace of Westphalia did not even unhinge the territories granted to Sweden from the Holy Roman Empire.
True, it did acknowledge the independence of the (Northern) Netherlands and Switzerland,
but these was just acknowledging the matter of fact of before the war.
The treaty did not even go so far as to accept the same fact for the Northern Italy territories.
But of course, in that epoche, you can have the de-facto situation you described,
with the formal integrity of France untouched.
To this end, a major portion of it has to be given as a fief, not necessarily to
a foreign ruler, but preferredly to an adversary of the king.
The main problem here is, as mentioned, the rather centralistic structure of France,
as opposed to Germany or even Poland or England.
However, the last polish of that political structure is relatively recent
(it was accomplished when Richelieu was already in power).
Although a French duke had not been as autonomous as a German one before that,
there will be still a lot of support for the idea of reenhancing the status of the nobility.
A lot depends on the closer circumstances you have in mind ...