Disable the space bar in settings key map
( and if you are worried about accidental operation disable all the other shortcuts as well)
Make a script cue triggered by the space bar with this script
tell application id "com.figure53.QLab.4" to tell front workspace
set thecues to cues whose running is true and q type is "audio" as list
if the (count of thecues) < 1 then
start (first cue whose q name is "Main Cue List")
end if
end tell
This checks to see if an audio cue is playing and if there are none, starts the main cue list (equivalent to a GO)
Mic