I was thinking of something like this too. I have an item I want to give away at a certain dollar amount. I was thinking though, of a button they can click once they are eligible. However, it also needs to have a facility to remove the item if they change cart contents and drop below the required amount.
I already have some things in place that cover part of this, and I may take a stab at it. I'll post back if I can work it out. Let me know if you find a developer that wants to tackle it too.