Thanks for the feedback, everyone.
I have changed the outhaul line for some spare line that I had intended to use as a back up for a control line on my Laser. It is a non-stretch white silky texture line that seems to be grabbed by the cleat very easily. I have not tried the boom in action yet, but once it is cleated I can't budge the replacement line by hand. If plan A does not work, I will pop rivet a clam cleat onto the boom.