MetronomeMark aligned to accidentals

[Daniel Rosen]

Consider the following example:

\version "2.19.3"
{
  R1 |
  \tempo "Tempo"
  <as' bes'>
}

I would like the MetronomeMark to be aligned with the left edge of the leftmost 
Accidental (almost with the BarLine, but not quite). I've been trying to 
manipulate the Score.MetronomeMark.non-break-align-symbols property, but to no 
avail (I'm not sure I quite understand how that property works; it's a little 
unclear to me in the IR). Can anyone steer me in the right direction?

DR