beyond what is already done in xmobar itself. There's also the question
of whether an X11-Xrandr package makes more sense (Xinerama is already
there, but xft isn't). I imagine xmobar would also pick up the code that
gets put into X11 or X11-Xrandr for its Xrandr needs instead of its own
bindings if/when that happens.
There's already an X11-xft package (which includes Render (not RandR) support; I'm not sure if that belongs there). As to RandR, my inclination is that it should be part of X11, not a separate package — as long as the package continues to work in the absence of RandR.