Is it possible to reduce MIDI gate lengths slightly (without resorting to using shorter lengths and rests)? Some of my external instruments have trouble receiving 'gate on' messages during continuous passages of notes.

With the :duration-add keyword you can shorten the duration of the note. The length in notation will not change.

 

(def-score duration-add
           (:key-signature '(c maj)
            :time-signature '(4 4)
            :tempo 120)
  
  (inst
   :omn '(t c4 d4 e4 f4 g4 a4 b4 c5)
   :duration-add '(-0.1)
   :channel 1
   :sound 'gm
   :program 'Church-Organ
   :volume 127
   :pan 64)
  )