I think this might be what you are looking for. I don't think it works but decaying keys but yet time locks that need to be renewed on a annual basis (I believe the wallet automates this). Maybe you will able to accomplish a lot of the same things...
Discussion
Have you used this wallet?
I tried it a year or 2 ago, seemed to work but I deferred digging any deeper at the time in lieu of other pursuits. There was another one too I think called MyCitadel?
I have "run the software" but not yet with any corn.
I am trying to find the best inheritance setup. And so far this is toping my list.