invisible pedal sustain

[Steve Yegge]

Hello,

I'd like to be able to add invisible \sustainDown and \sustainUp
instructions to a guitar piece I'm working on, so that the MIDI
output uses the sustain, but no sustain stuff appears in the score.
(For these arpeggiated chords, the sustain is implicit.)

I tried

  \override SustainPedal #'transparent = ##t

with no luck.  Any suggestions?

Thanks,

-steve