[Frumious Bandersnatch]

Where does the Constitution outlaw meetings of state delegates?

They don't, but there are those who indicate that a constitutional convention of slaveholding states was not discussed officially until after each state seceded.  Such was not the case, and in any event, such a constitutional convention would not have force of law since it did not follow the form prescribed in the constitution.  Expecially, since the slaveholding states pointedly (and illegally) excluded the northern states.

[stainlessbanner]

I see no reason delegates could not meet in Montgomery to discuss political affairs.

The states that already seceded (in January 1861) sent delegates to the Convention. These states, including South Carolina, Mississippi, Florida, Alabama, Georgia, and Louisiana were effectively independent, sovereign states.

Virginia, Arkansas, North Carolina, Tennessee, Missouri, and Kentucky were not represented at the Convention, along with other Northern states still in the Union.