I guess I'll be the contrarian yet again: I kinda like the idea, at least in principle. Our Congress has become extremely dysfunctional so that even basic necessities like raising the debt ceiling become epic power struggles with the real possibility that Congress could fail to raise it. "Fast tracking" is a good way to help reform that process. Presumably the recommendations of the Super Congress would be at least somewhat representative of Congress as a whole, since they would be well aware that they had to get the support of their colleagues for any changes they were making.
As for the constitutionality/legality...there is no reason this would be unconstitutional. All laws would still have to be voted on by the entire House and the entire Senate...they just wouldn't get a chance to add amendments and continue haggling long after the bill had been written by the "Super Congress." Instead, it would be subject to an up or down vote. Allowing amendments to bills are not a constitutionally-mandated feature of our Congress, nor are they even federal laws. They're just the parliamentary rules by which our Congress has agreed to operate, and can be changed at any time.
As for whether or not I'd ultimately support it, I guess it boils down to the specifics of the proposal, including how its members are chosen. Does the leadership just pick whoever they want, is it the leadership itself, is it based on seniority, is it random, etc.