IIRC, Twitter's approach was to also give the inventor a transferable license to the patent. That way if Twitter used it offensively, the inventor could independently license the patent to the company who was being sued.
That's very reminiscent of the Spooner vs. Tucker argument about dual ownership of a patent; Spooner conceding to Tucker that co-inventors should each be able to freely license, then Tucker asking if one inventor were to give away the license to the world for free would he be infringing on the other inventor's rights.
Yes, they could hand over the patents to RPX, and pay the annual fee; instead of 'as protection' it would be 'as a donation to the public'. Perhaps an odd use of RPX, but it would be really effective.
Is there anything they can do that's binding outside of a declarative 'we won't sue you unless...' statement?