I have found an algorithm that is working well to restore transforms’ attachment parent on load.
This is a long-time issue I couldn’t figure out how to do without making the loading performance a slug.
I couldn’t find previously a way to restore them without assuming the parent Actor still exists, but the new code seems to work fine for that;
If there’s no parent, transforms are now registered in world space, otherwise plugin now records a transform relative to parent.
I’m submitting update 2.9.5 with this change, for Unreal 4.25.