Alec..

You really have to upsize the cleat for a one line system. The spinloads resolve to that one cleat. I wound up going to the largest Harken cleat and then wound up with double cleats in series. In the end... I found the seperate tack and halyard with double midsize cleats on the halyard were the best solution. Another thing that can happen is that the cover and core of your halyard seperate at the cleat point and then the cover fails and you can't keep the chute up. (been there... done that as well)

IMO, I would not reinvent the wheel here.

Mark