In most vessels with a jib set above the normal jib and staysail, the sheet would go to a fairlead, attached in a suitable place, on the standing rigging, then down to the deck. This was the practice in Thames barges for instance. Even in a staysail barge, ie no bowsprit, the light weather sail set high on a stay from the top of the mast to the stem head would be sheeted in this way. The same would apply in a bowsprit barge, but with the stay going from topmast head to end of bowsprit.