How do I estimate the mass of a deciduous tree's leaves?
David LeBauer is correct in saying that there are 'allometric equations' to estimate the leaf biomass, but they only refer to small experimental plots in such places as Hawaii, Japan, Singapore and Malaysia. The number of species involved is a few dozen at most. As 'N9ZN' points out, trying to develop generalizations would be a titanic task, and essentially impossible without a vast amount of experimental work. There are about 50,000 tree species in the tropics alone, each with its own calibration curve, not to mention ecologic, microclimatic and nutrient constraints upon leaf development. 

I would suggest that, instead of trying to develop a deterministic result, since leaf biomass is approximately related to potential evapotranspiration and water availability/constraints, it would be easier to estimate average water usage by stem dendrometry, crunch the numbers for ET0 based upon climatic data, and then develop a biomass conversion efficiency. As far as I know, this hasn't yet been done, at least on anything other than a very small scale - but theoretically it should be doable.