Hi @naiad ,
unfortunately, you cannot directly set a custom axis (afaik), but the axes used in Symmetry and Locking are the Local axes of your Object:
so that might help. Also, you can keyframe different rotations so that you can easily switch the orientation of those axes.
Watch out: if you use the Transform or Rotate Tool, that doesn’t rotate the Object, so the Symmetry and Lock Axes won’t be affected by that rotation!