Make a custom alias and use that with /play -a.

/play -a shortplay chan myhugefile.txt 1500

Code:
alias shortplay {
  inc %shortplay
  set -u10 %shortplay %shortplay
  if (%shortplay < 5) { msg chan $1- }
  else {
    unset %shortplay
    play stop
  }
}


That's just a quick example and could probably be improved. It's just to give you an idea.

Note that the set -u10 line is there just to prevent the variable from staying set if you play a file with under 5 lines.

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