Separate 'tip wallet' that uses posting key is a great idea I think. Since it will only be limited to the 'tip wallet' and users will have to use an active key to transfer to tip wallet I don't it would be risky. Users can choose not to keep anything in the tip wallet, or keep very little. I think this would be a great addition. This way content can be rewarded even after payout.

I was suggesting changes in Steemit UI level though since Steemit has been making UI changes like downvote button, edited, Steemit roles, featured, etc. What I had in mind was a 'tip' button that would redirect to steemitwallet.com with filled form ready to send. Of course not as simple as using a posting key, but something that can benefit authors, especially if author rewards will be going down to 50%.