Prox c. has a screamin' hot proper motion, as you'd expect from it's proximity, so it's moving across the celestial sphere at a pretty good clip.

The real problem is that Prox c. b is only 0.04 AU out from its host star. So the absorption spectra for a star lined up with the planet is going to be pretty well contaminated with light from Prox c. You could imagine various schemes for moving around the observer for the best angle or big occultation disks, but at a certain point it's going to be easier to just fly a probe over and sample the atmosphere directly.