[osteoderm]

David,

It sounds like you have a jackyard tops'l, the main benefit of which is that there is no standing topmast when the tops'l is struck. With a short masthead and long(ish) yard, the other variant is a Cornish Yard tops'l, in which case the entire length of the tops'l luff sets above the gaff, and is lashed to the yard; the tackline is then properly called a "trimminoggy" and and runs from the heel of the yard, through a fairlead on top of the gaff jaws, and then to deck level. i'm guessing that you have the other (more usual) sort of jackyard tops'l, however, with the luff extending below the gaff a little ways, and lashed to the yard only over its upper reaches.
The common and traditional way to rig the tops'l sheet (or clew outhaul, as you describe it) is as you've assumed; from the clew to a turning block at the after end of the gaff, then forward and down to a turning block on the lower side of the gaff jaws, continuing down to deck level to be belayed. Traditionally, the belay point is on the main boom, near the gooseneck/yaws, and the turning block at the gaff jaws is hung from a short pendant. With the belay point on the boom, sheet tension ought to remain constant from one tack to the other (as opposed to being belayed to a pin on deck, where the lead would change).
The reason for the short pendant up at the gaff jaws eludes me, however that's almost always the way i've seen it done, and continues to be the way i rig my own tops'l; i'm assuming that there's some good reason involved! Perhaps it's to help prevent a flogging sheet from getting caught on the jaws, or just get the block a little further aft of the mast for a cleaner lead downwards.
Running the sheet this way is probably easier on the gaff than running it from the gaff end to deck; the loads are close to straight compression on the gaff. In fact, it's unusual to have the sheet run as you have it now rigged. The only reason to run the tops'l sheet straight down from the gaff end is to use the sheet as a gaff vang to prevent the gaff from sagging to leeward. In which case, as you've noted, the sheet must be run down on the windward side of the mainsail. With the block at the gaff jaws, you lose this ability with the tops'l set.
A caution: i've played around with gaff tops'ls a fair bit, on boats of various sizes, and i've learned that on traditional boats, "easy" and "simple" are definately not synonymous. A gaff schooner certianly doesn't come all that close to "easiest, most convenient"!
As far as further reading/reference goes, i strongly reccomend John Leather's "Gaff Rig Handbook".