Dan McCall (Georgetown 2005) has published Are There Added Preservatives in Section 170(h) of the Tax Code?: The Role of Easements in Historic Preservation, 39 Real Prop. Prob. & Tr. J. 807 (2005).  Here is the abstract:

Owners of historic property are able to take tax deductions when they donate preservation easements. This Article discusses how to determine the value of a preservation easement and concludes that the value of the easements typically is not as great as commonly believed. Because the tax deduction value and the actual value of the easement are often different, any type of realignment between the two values likely would reduce the number of donated easements. The author concludes that realigning the values, which would treat the easements as commodities, might reduce the efficacy of easements as a preservation tool.