Topping Lift Ramblings
I've put a stopper knot on my topping lift tension/slack line in a spot where it can be let slack when the mainsail is up, but the knot will jam up against (in my case a block) to prevent the boom from ever falling down to head height in the cockpit the event that mainsail is dropped without first securing the topping lift line.
When sailing, I'm frequently fiddling with the topping lift play so there is some slack, but its not flapping up against the sail and banging up against the backstay. For instance, if I have just the right amount of topping lift slack when I have the mainsheet and/or boom vang hauled in tight, when I ease the mainsheet out for say a broad reach and the boom raises (which it does a bit even with the boom vang still tensioned) the topping lift becomes very loose, and needs to be taken in a bit. Going the other way (from mainsheet let-out to in-tight), the topping lift then needs to be let out again, or the mainsail leech can't be made taught.
It was suggested by a sailing neighbor, which I have adopted, to add some stretch cord in tandem along the topping lift's last couple of feet before it intersects with the boom end. Note this cord added in tandem to the topping lift. The topping lift remains connected to the boom as normal. Adjust the stretch cords so it has a little tension. It will then stretch out when the mainsheet is pulled in tight, but will still keep some tension on the topping lift when the boom raises as the mainsheet is let out. Less adjusting is needed when sailing.
With all the fiddling demanded by the topping lift, guess that's why rigid boom vangs are generally preferred these days... But I like fiddling, so I'm staying with the topping lift for now.