Thx Noors! I meanwhile stumbled upon the script you linked. Just this moment I was tinkering with deleting the symmetric half and capping the resulting hole. Then I used the script to place a working pivot on the resulting cap-plane which gives me a working pivot whose Z-axis is the symmetry planes normal. Sadly I could…
I had a pauldron rotated and attached to a curiass and detached it later on. The pauldron itself is symmetrical but due to the attaching and detaching the original pivot got lost. I made some changes to the pauldron and now need to symmetry (modifier) it again. Rotating the symmetry plane is tedious but kinda works for…