Main Top Sail Bow Line

The main topsail bowlines probably belay at pins in the caprails along with a lot of the other topsail gear. There were probably round fairleads seized to the shrouds to guide them (we have plenty of spare fairleads, nice little lathe-turned items). Your lead, to the aftermost fore shroud, is a logical option. From there, the line would follow the shroud down to the rail, possibly going through a fairlead somewhere above the fiferail.

In general, very little of the rigging is belayed in the middle of the ship, most of it was led to the caprails.

Fred