I figured out a workaround how to effectively change the offset at runtime, which allows me to switch door directions or make them swing either in a single or both directions:
Image: Two instances of the same Blueprint, the one on the right has no offset and will swing both ways, while the one on the left is only allowed to swing in one direction.
Essential for me was to have the mesh remain at the same location so the Blueprint can be easily used as a drag&drop component for artists to place into other Actors and be able to reliably tell how the movement will behave.
The workaround is to apply an offset to the relative rotation of the mesh before constraining it, then reset that offset.
What is that performance assumption based on?