Would like to propose the solution for sharing the private key / other sensitive information with a modified SSS scheme. 

The key features are:

1) the secret restoration is not possible without corresponding signatures

2) share[i] belongs to privateKey[i] only, which means it can't be signed with a different private key

I hope this approach can help on the application level to share sensitive information or it can be used as m-of-n multi-signature (this is discussable). I've attached the pdf file with the concept description. Also here is the repo with a working example in node.js: https://github.com/ega-forever/esss

Thanks in advance for your time.