for posterity this feature is now implemented in slicer, and part of the CropVolume module since Sept 21s.
To align your volume with anatomical planes, hit expand “reorient volume” tab, then use the widgets to transform your volume and harden it to finalize. If you want to create a new volume that’s in inline with the world axes, choose the Align to world axis and resize from the new drop down associated with Fit to Volume button. This will resample the oriented volume into the new grid and save the output to the output volume.