It can be the amount necessary to cover taxes, but can't exceed the amount of Federal, State, local, and foreign taxes required to be withheld in connection with vesting. Treas. Reg. 1.409A-3(j)(4)(iv).
EBECatty raises some very good points, but the design does make it look a lot more like a retirement plan, e.g. if the plan provides for a 10- or 15-year payout, in approximately level amounts, following separation from service. Income tax on all the earnings is deferred, no more FICA is owed, and most of the payments (e.g., over the 10- or 15-year period) are tax-free recovery of basis. For a senior executive and board who want a real retirement benefit, it's a good design. The biggest issue is the creditworthiness of the 501(c)(3), so makes the most sense for a well-established employer.
And finally, explaining how it works makes for an interesting PowerPoint presentation, if you're into that.