Not quite that early, but very close to home, is the following brief statement that was signed on June 4, 1775, only fifteen days on the heels of the "Mec Dec":
The actual commencement of hostilities by the Britsh troops in the bloody scenes April, 19 last, was Boston, the increase of arbitrary impoisitions from a wicked and despotic ministry – and the dread of instigated insurrection in the colonies are causes sufficient to drive an oppressed people to the use of arms.
We, therefore the subscribers, inhabitants of South Carolina holding ourselves bound by the most sacred of all obligations, the duty of good citizens toward an injured country and thoroughly convinced under our present distressed circumstances we shall be justified before God and man in resisting force by force, do unite ourselves under every tie of religion and of honor and associate as a band at her defense against every foe; hereby solumnly engaging that wherever our continental or provincial counsils shall decree it necessary, we go forth and be ready to sacrifice our lives and fortunes to secure her freedom and safety. This obligation is to continue in full force till reconciliation shall take place between Great Britain and America, upon constitutional priniples, an event which we most ardently desire and we will hold all those persons inimical to the liberty of the colonies who shall refuse to subscribe to this association. 
Not quite as distinct as the Mecklenburg document, and holding out a hope of reconciliation which the Mecklenburg signers didn't, but drawing a line on principle nevertheless.
We may have a family connection to the Mecklenburg Declaration -- several of the signatories were named Alexander, and there are Alexanders in the Smith side of the family though all the relationships haven't been sorted out yet.
However, the declaration from South Carolina was signed by several direct and collateral ancestors on the Young side --
- Four named Singleton, including Matthew, John, Joseph, and Robert Singleton (my 6g-grandfather);
- Josiah and Caleb Gayle (likely brothers of Robert Singleton's wife Sarah Gayle); and
- Thomas and Richard Bradford (thought to be my 6g- and 7g-grandfathers; Thomas Bradford's son Richard married Robert Singleton's daughter Elizabeth).
- There were also Wrights and Rodgers in the list though I'm not sure about them.
So if Charlotte has its May 20th Society, perhaps the Youngs can celebrate June 4th.
 McCrady's South Carolina, vol.,II, p. 792; quoted in Hugh Charles Haynsworth, Haynsworth-Furman and allied families: (including ancestry and descendents of Sarah Morse Haynsworth) (Sumter, SC: Osteen Publishing Co.., 1942), 39-40.