Wouldn't state legislatures still have a say on the rules for the state ratifying conventions, if Congress chooses to go that route? Each state would choose its own rules for their own conventions, too, right, because these are state-managed events.
Would the federal government try to argue that "equal protection" allows them to insert themselves into the state processes to make each and every state hold their conventions in exactly the same way? Today, "equal protection" does not allow the federal government to intercede when some states hold primary caucuses and other states hold popular votes, so why would the federal government be allowed to dictate how a state holds its own ratifying convention?
-PJ
The states held special elections for their conventions by having one (or more) delegates elected from each district of the state's lower house of the legislature.