my scenario would be for ships built on Admiral Hipper-class, they would need a tanker for extended operations and helpful if they could launch and/or retrieve seaplanes. (think the high pressure steam, at the time, was a reasonable gamble)
(btw that is good idea for towed tank, similar or same design schemed to carry V-2?)
That was part of that original proposal as mentioned in Rossler THE-UBoat..pp 207 []along with Kohr's transport U-boat proposal]., to use off the shelf technology.