Great idea, but the lawyers would shoot it down immediately (if they haven't already). The terms of a settlement would just have a clause preventing them from offering such a bounty.
So just do it ahead of time. As soon as the troll approaches you, before you so much as sign an NDA, immediately set up an independent trust with the money in it which will give it to whoever invalidates the troll's patents, and if the patents expire without being invalidated the money plus interest goes back to the company. Do this immediately, for every troll, by every company. It could put a pretty good sized dent in the problem.
.. so then as part of the settlement, the trolls says 'and withdraw your bounty'. Same thing.
Hence the words "trust" and "independent" -- you set it up so that it's totally out of your control. Then you can't withdraw it any more than you could prohibit the EFF from doing the same thing if they chose to.
Every C&D can be posted to a Kickstarter-like site, which collects for the pool of funds. Other companies who receive the same C&D can join the group, basically creating a class-action in advance of the civil action being filed. Potential trolls could look at their prospects and make decisions thereby in a form of mutually-assured destruction (patents on the trolls part, the funds on the receiver's parts).