I think there’s only one sheave at the top so you’d need to fit a cheek block to the outside of the masthead if you wanted another line up there. My boat has a little one for a burgee halyard. yes I know, its sweet!
I’ve only used a topping lift when sails are furled, when moored or motoring, and the main halyard does a lovely job of this. Most boom ends have a clip you can pop the line into, or you can loop it through the outhaul. Modest tension in the kicker and main sheet (centre) seemed enough to stabilise the boom side/side (mainly I think through the tension transferred into the bridle). I’ve also heard other folk talk if using the spinnaker halyard for the same purpose: your mast has (or should have) sheaves at the hounds and a third sheave at the foot for that, even if you don’t have a spnnaker halyard rigged just now.
probably like everything else in wayfaring there’ll be several different ideas on this one! Also depends on what you want it for.
Boris – Delphy W6330