This isn't pro bono work. Pro Bono work is usually performed at no charge. The law firm makes a certain number of lawyers available for free and doesn't charge for expenses like photocopying, postage, service, and travel, which can be considerable. Pro bono work usually isn't open-ended. The law firm takes on discrete matters that can be resolved in a reasonable amount of time like a year or two, often less. Often law firm associates (as opposed to partners) are assigned to pro bono cases and in many large law firms with up-and-out systems, the associates leave after a few years to work somewhere else.

I don't think many competent lawyers with this level of expertise and experience would want to work for years on a project of this type for less than their normal fee. It doesn't seem to be very rewarding work and it appears that the lawyers don't have the power and political support to accomplish all that they need to. If you're going to work at less than your regular rate you want the experience to be fulfilling, to be good training for your associates, and to generate good PR for your firm.