In that case run the downhaul out just shy of fully detensioned. I personally have never had the mainsail unhook on me.

But then again I pretension my downhaul system. Meaning that even when both cleats are completely released the travel length of the downhaul itself it limited to a state that still put some tension on the luff. This is handy on a spi boat. The pretension setting is just enough to get the wrinkles out when there when the mainsheet it only mildly tensioned (= downhill setting in light to medium winds)

Wouter