Parma was initially supposed to be hereditary for Marie-Louise, to pass to Napoleon II upon her death until Napoleon's return from Elba and the One-Hundred Days. The Congress of Vienna then decided she would only hold it for life. Prevent the One-Hundred days and Napoleon II will succeed his mother in Parma.
Of course, if thats the only PoD, Nappy-2 predeceases his mother

And I think his heir would be Nappy-3 as well, and that one surely will not simply stay in Parma. I guess we could have a double PoD, or if we assume a (n even more) Habsburg Nappy-2, a triple PoD.
The problem is, even if Franz I wanted to give him a throne, how would it be received by the Bourbons in France and by Britain (and everybody else)? He can argue that the boy is more Habsburg than Bonaparte, but everybody else will see him as the soon of the Corsican ogre.
Well, if its just a return to the pre-100 days solution, then there oughta be some wiggling room. Especially since the Habsburgs clearly will not accept that Nappy-2 is just regarded as the Corsican Orge Mk2. The real problem I see is that after 1815 it would indee dbe cheating the Bourbon-Parmas and that has potential for an international crisis - again, unless we take the double/triple PoD solution.
Also, if he is installed in some minor throne, how can you convince any crazy revolutionaries (from France, Poland, Italy, wherever) not to go there and invite the "son of Napoleon" to be their ruler? IOTL even Brazilians wanted to help Napoleon to escape from Santa Helena in order to have him as their ruler. Imagine how many people would try to convince Napoleon's free son. You would need him to be a very humble and weak man in order to refuse so many invitations and not care for people saying his name on the streets.
Hm. Of course, Bonaaprtism wasnt just a dynastcial issue - it was also ideological. So, lets see Nappy-2 does win the French throne, only to disappoint anybody to govern in a, well, a Habsburg style. I think hed be toppled again, and that was that then for him...