R.C. Anderson's The Rigging of Ships in the Days of the Spritsail Topmast addresses this question - though not thoroughly enough to give a straightforward, definitive answer.
Pp. 76-77: "...Unfortunately there is no doubt that the knights were often between decks. Van Yk, writing in 1697, about Dutch shipbuilding, says that the knights in men-of-war were on the lower deck of a 2-decker and the section of Allard's book of 1705 shows the main knight in an almost incredible position on the lower deck of a 3-decker. A French section, of 1691, shows the knights on the middle-deck of a 3-decker with both uper deck and forecastle above the fore knight, but the Royal Louis model, of similar date, has her fore knight on the forecastle and the main knight on the upper deck with only the half deck over it. At an ealier period (about 1650) the Swedish Amarant has the fore knight on the upper deck and the main on the lower, while the Danish Norske Love has her fore knight on the forecastle and the main probably on the upper deck.
"The position of the knights is naturally associated with that of the capstans. To follow the matter further would lead into all sorts of side-issues only remotely connected with rigging. There is little doubt that the knights of Dutch ships of the 17th century were more often than not between decks and the same would probably hold good for English ships in the first part of the century, when they were using such fittings."
Pp. 78-79: "After the lower halliards and the top-ropes, the topsail sheets have to be given a lead and a place for making fast. They went to the bitts, which were a pair of timbers rather like the knights but smaller, which stood just before the lower masts, one on each side. At first they were probably simple uprights with a sheave in each...but later - somewhere about 1660 in English ships and later in foreigners - they were connected by a cross-piece....About the same time, when English ships took to double jeers instead of ties and halliards, a second pair of bitts, with a cross-piece, took the place of the knight abaft the mast and provided for the hauling part of the jeers....
"The sheet-bitts and jeer-bitts were sometimes nearly as inaccessible as the knights. The model of the Prince, of 1670, has the fore bitts beneath the forecastle and the Dutch section of the end of the century shows the main bitts on the middle deck of a 3-decker. By 1700, and probably before that, English ships had raised their fore bitts to the forecastle...."
If I'm reading that correctly, he says the bitts forward of the mast would be for the topsail sheets and the ones aft of the mast for the jeers. I wouldn't put belaying pins in the cross-pieces unless I had extremely impressive evidence for them. But I think a big block, hooked to an eyebolt in the deck next to each bitt, would be appropriate as a means of taking the line to the capstan.