Quick update: after 66 years it has finally happened, the claim has been fabricated.
I happen to agree with jpinard, there should be a mechanic where the odds to successfully fabricate a claim increases after a while. If you're unlucky, as I have been, it can literally take generations to succeed. While it may project some realism, I feel that having to babysit your single county for 50+ years is getting old real fast. Maybe there should be a mechanic where fabricating claims is faster/more successful for those with a smaller demesne and slower once you grow to a certain size, but not make it impossible. In the end though, it's still luck of the draw.