No there is no reason why secession should not be possible, with the consent of the people. The southern acts of secession failed to take that into consideration.
Interesting that you know that, considering no records were kept of the proceedings, owing to their arguably treasonous nature....or are you saying it wasn't treason?
On the contrary, records were kept. The Hartford Declaration is available on line, and any reasonable study of the convention will confirm that those desiring a separation from the United States were a minority, and were voted down very early on.
In fact, were not representatives dispatched to Washington to negotiate such, in addition to the declaration/proposed amendments to the Constitution?
No.
Besides, Aaron Burr was part of a secession attempt nearly a decade before Hartford, as a result of the Louisiana Purchase.
And he stood trial for it.
"The People of Virginia declare and make known that the powers granted under the Constitution being derived from the People of the United States may be resumed by them whensoever the same shall be perverted to their injury or oppression and that every power not granted thereby remains with them and at their will." --Virginia's ratification conditions for the Constitution, 1788.
But when they choose to act outside the bounds of the Constitution then they are entering into rebellion. And that only works if you win.
"In fact, were not representatives dispatched to Washington to negotiate such, in addition to the declaration/proposed amendments to the Constitution?"
You said...."No."
Actually, the delegates were sent to Washington.
"A constitution for the New England Confederacy was drawn up, and it was stipulated that it would go into operation 'as soon as two or three states shall have adopted it.'
The Convention even designed their new nation’s own flag. The flag contained five stripes, representing the five states of the proposed New England Confederacy.
Moderates in the convention convinced the delegates to propose changes to the US Constitution instead of immediate secession. A special report with recommendations was drafted.
This official document of the convention was dated December 15, 1814. The final report arraigned Madison's administration and the war and proposed several constitutional amendments that would redress what the New Englanders considered the unfair advantage given the South under the Constitution. If these changes were not implemented, the Convention recommended secession.
By the time the Hartford delegation arrived in Washington to make their recommendations, the War of 1812 was over.