I was looking for something else yesterday in David P. Currie's The Constitution in Congress: Descent into the Maelstrom 1829-1861 when this grabbed my attention (emphasis added):
Most notably the [March 1, 1845 Joint Resolution for annexing Texas to the United States] provided that Texas might at any time divide itself into as many as five separate states. . . .
No one objected, moreover when the resolution was amended to require that slavery be prohibited in any state so admitted from [Texas] territory north of the Missouri Compromise line of 36 degrees 30 minutes. The compromise applied only to areas acquired by the Louisiana Purchase; the Texas provision extended the line westward. Not only did the proviso merely reassert Congress's purported authority, exemplified by the compromise itself, to ban slavery in the territories; it imposed a condition on the admission of new states. Southern apologists for the extension of slavery would soon have the unenviable task of explaining it away.
Section 2, Third of the Resolution included the provision that allowed the creation of up to four additional states, in addition to the state of Texas, to be formed out of Texas's territory with its consent:
Third. New States, of convenient size, not exceeding four in number, in addition to said State of Texas, and having sufficient population, may hereafter, by the consent of said State, be formed out of the territory thereof, which shall be entitled to admission under the provisions of the federal constitution.
The same section goes on to provide, as Prof. Currie points out, that slavery "shall be prohibited" in any Texas Tots (as I like to call them) that may be created:
And such States as may be formed out of that portion of said territory [of Texas] lying south thirty-six degrees thirty minutes north latitude, commonly known as the Missouri compromise line, shall be admitted into the Union with or without slavery, as the people of each State asking admission may desire. And in such State or States as shall be formed out of said territory north of said Missouri compromise line, slavery, or involuntary servitude, (except for crime,) shall be prohibited.