Eh it's a good idea, but lends itself to some really annoying situations. Say you locked content with a contract to unlock it with a 5 cent upvote. 5 cents is not a lot, but a 5 cent upvote currently requires thousands of staked coins.
Clearly, you'd have to be able to unlock content with either upvotes or liquid tips. I suppose the logistics of such an implementation are trivial. Just trying to play out this thought experiment.
https://steemit.com/steemworld/@edicted/encrypted-memos-are-unreadable
I'm suddenly reminded of this post I wrote 3 months ago. This could be a great use of the memo key... of course once a single person unlocks content they could just repost it unlocked. It would theoretically only take one whale to undermine the entire system.
It's a very soft-consensus / honor-bound solution, but if done right, it could likely work out quite well, regardless of the shortcomings.
The thing with voting is that it really isn't a heavy cost, so you would expect a lower exploit rate than traditional media. So while still soft-consensus, there is a mechanism in place that could encourage better behavior while requiring some "effort" to exploit.
There would be some mechanisms that you would have to play around with that either requires a hardfork or and third party facilitator in order to implement properly and smoothly, but as you mentioned, the underlying encryption ideas are there to borrow from.
I think that most people on a front end with these implementations would play by the rules, even though circumventing them might be trivial for some users.