I have given a look to the code and perhaps the man page is old.
`mdelay' writes the midi status byte in an internal buffer only when

     status = 144 (note on)
or   status = 128 (note off)

and sends a midi note message after the delay (when the read value of
the buffer is non-zero, so 144 or 128). Here is a variation of mdelay.csd

<CsoundSynthesizer> 
<CsOptions>
-odac -iadc -d -M0 -Q1
</CsOptions>
<CsInstruments>

sr = 44100
ksmps = 10
nchnls = 2

massign 0, 1

instr 1

  kstatus, kchan, keynum, kvel midiin

  idel1 = .2
  idel2 = .4
  idel3 = .6
  idel4 = .8

  if (kstatus == 128 || kstatus == 144) then
    midiout   kstatus, kchan, keynum, kvel
  endif

  mdelay    kstatus, kchan, keynum+2, kvel, idel1
  mdelay    kstatus, kchan, keynum+4, kvel, idel2
  mdelay    kstatus, kchan, keynum+6, kvel, idel3
  mdelay    kstatus, kchan, keynum+8, kvel, idel4

endin

</CsInstruments>
<CsScore>
i1 0 36000
</CsScore>
</CsoundSynthesizer>

tito