MAPs and scale lengths with APOGEE RC *'s

Had conversation with Bovy what the trial and tribulations might be to
do MAP scale lengths using the RC stars from APOGEE:

simplest version:
-- focus on apocenter quadrant (bearable reddening)
-- each star in the RC catalog has a distance and a reddening attached to it.
-- to the extent that RC *'s are standard candles, and enter the sample down
   to the apparent magnitude limit, the selection function/volume issue
   should be analogous to the unreddened case, except for exp(-tau_H)/D^2
-- focus on stars at +-4 degrees ftp, the Galactic plane --> don't care about scale-height.


simplest science question, what is R_exp([Fe/H] | [a/Fe]~0)?
is it flat for the metal poor stars?