This is a great idea, but...
The Posting Key will be given to the participate to conduct their AMA
Don't do this
.
With the Steem API being so flexible, you can easily get some cool and smart person (ahem) to code you up something simple to redirect their access.
It is always far too risky to share keys, even among friends and family! On the blockchain key access is account ownership.
We've already begun!
https://steemit.com/life/@askme/i-m-jamie-your-transgender-temporary-girlfriend-ama
Will certainly be taking your suggestion into account for future AMAs!
Nooooooooooooooooo!