Welcome to Flyertalk, and welcome to the BA forum willoL, it's good to see you here, and I hope we will see more of you in the future.

Having Googled that bag (seems incredibly expensive!!!!) I think it should go in to an A380 wardrobe, particularly if you didn't have anything hanging on the rack too, though it may have to go in sideways down. I am doing this from memory, but this wardrobe, which is opened from the aisle, is about three quarters of the unit depth, since the TV gubbins on the seat side take up some space. As you say, there is a ledge where you could fold the jacket to create space below the ledge, and that goes all the way down.

If you wanted to be totally certain of something that would work, then perhaps consider one of those upmarket all purpose jute or hessian bags, since they work very well in that locker. But to re-iterate the comments made elsewhere, so long as it wasn't too heavy and so long as you approached the matter correctly, I think you would get assistance from the cabin crew, particularly given your stature. I suspect the crew would prefer to help than see you standing on a seat. There may be exceptions, but I really wouldn't worry about it, despite your previous bad experience, most people most of the time are mostly lovely. I hope you have a good flight and can report back on a good experience.