Transit Depth Color Versus Transmission Spectroscopy
But this is already possible. Spitzer has performed "low-resolution" transit spectrophotometry of several exoplanets in its near infrared bands - see for example Tinetti et al. (2007);  Knutson et al. (2011); Desert et al. (2011).

I dare say you could get better data on more planets with a dedicated telescope with a larger mirror and the ability to image in several bands at once. All that is required is money.