What specific legislative proposals were the basis for Jefferson’s claim that George III “prostituted his negative” by vetoing them, so as to continue the slave trade?"
King George issued decrees such as this one:
"upon pain of the highest displeasure, to assent to no law by which the importation of slaves should be in any respect prohibited or obstructed"(Source)
Jefferson was not the only Founder who recognized the King's role in preventing the colonies from abolishing slavery. At the Convention, George Mason opened one of the debates this way:
"Col. MASON. This infernal trafic originated in the avarice of British Merchants. The British Govt. constantly checked the attempts of Virginia to put a stop to it."(Source)
That's why I found Adams' words so curious. I'd bet that the majority of the Founders held the same view.(whether they wrote it or not is another story)
"On the surface it seems the grossest hypocrisy for Jefferson to overlook the role of American colonists in creating demand for slaves."
That's what makes the king's role so profound. As you pointed out, the crown was involved with slavery from Elizabeth and then in the Royal African Company.
If the American colonists had demand for slaves, who created that demand for slaves in the first place? It all goes back to the king.
Furthermore, if the king was vetoing measures, that means the measures were passing. So the colonial legislatures, listening to their constituents, had many more voices for liberty than they had "entrenched interests" for slavery. But the king, in vetoing such measures, was helping create even more entrenchment.
Thanks you for the information and links. The Bancroft info provides clues on time and place that do help narrow my search for the texts of the actual measures. Mason’s 1787 comments, as well as the Pinckneys’, et al., highlight the critical difference in Virginia versus SC/GA motives: Virginia’s superabundance of slaves relative to their direct utility as laborers, and the SC/GA perceptions of slaves as a saleable commodity. Antebellum politics regarding the Westward expansion of slavery can’t be understood without the later.
I am curious whether other colonies than Virginia were passing measures to curtail importation.
On the question of demand — I have a hard time casting the early large scale plantation owners as victims of royal slave pushers, or of separating slavery from the entire colonial enterprise (excepting the religious colonies). Potential exploitation of vast land patents begged for labor, fueling the demand for slaves. Those colonists buying and holding slaves on a large scale were mostly British subjects (some Dutch in NY), often sons of aristocratic families, granted enormous land patents through their high connections. They were producing products, and buying them, within the English market in the early period (say pre-1740), as a captive arm of the British economy, as the crown might have hoped. As part of a successor generation, in a way saddled with slaves their forefathers had acquired, Jefferson understandably would like to blame George III alone for a problem set in place long before his time. The complaints about GIII blocking non-importation in the late colonial period are disingenuous, and in my view, were appropriately dropped. The Declaration needed to be free of debatable points.