Ok, figured it out with the tow truck driver. It's a lever above and slightly to the right of where the parking brake is, i.e. up under the dash. See the for sale section in a few minutes for pics of the move.
Just had the Imperial loaded on to another flatbed tow truck to send it on its way. I snapped a pic of the release lever in case anyone else ever needs to know about this. It's the rust-red horizontal lever partially obscuring the fuse box. Push it to release the e-brake, doesn't take very much effort.