I have done a bit of work with steam, you are right that it would soften the plys up, I would be afraid the area you are steaming would wick the steam up, you may end up delaminating more than 1 ply, your drum may also lose its structure and fall out of round.
Thank you, yes I was concerned about the steaming creating more problems as well. Thanks for adding your opinion. One thought was to slide a piece of aluminum coil stock under the warped veneer, steam it up, clamp it down flat then let dry. If it stays flat then start gluing. But then again the ply will compress flat fairly easy as dry as it is so Im not sure if its worth the risk wit the steam or not, probably not.