I think you’re hitting an inherent limitation of the character movement code at the moment, which is that the capsule is always vertically aligned and that the “floor”/“base” is always down.
If you want to try to work around it I think you either have to implement a custom mode, use a mode that is not PhysWalking, or change a bit of the current walking code to remove the vertical assumption and change the floor traces to trace relative to the capsule orientation. Other things like IsWalkable() would also have to be changed… it’s not a small task 
Something more scripted (matinee) is possibly the best option for now.
We’ve started an internal discussion about the possibility of relaxing these restrictions, but I can’t promise any sort of ETA or outcome at the moment. This thread has certainly been keeping it on our radar though, so thank you!