Surely this is yet another untold tale? With Superman and Batman teaming up regularly, and Batman having his own secret untold tales with the Legion (to be discussed in an untold Legion World thread), there were plenty of instances of the Legion and Batman being involved in each other's cases.
Such as the likely Clayface tricking Supes & Bats, nearly beating them, with Chameleon Boy (and Proty!) bailing them out!
So...would this be a flashy type story with multiple crazy shape changes: dragons, rocketships, etc.? Or would this be a subtle bit of spy versus spy? Or both? Complete with what appears to be Superman versus Batman? (Thereby explaining the inevitable Superman / Batman War cover).