Pengelly et al. 2010 used a Scanalyser growth imaging system (LemnaTec) to estimate plant volume of individual plant, I think it should be able to estimate leaf area as well.
Pengelly J.J.L., Sirault X.R.R., Tazoe Y., Evans J.R., Furbank R.T. & von Caemmerer S. (2010) Growth of the C4 dicot Flaveria bidentis: photosynthetic acclimation to low light through shifts in leaf anatomy and biochemistry. Journal of Experimental Botany, 61, 4109-4122.