We’ll implement ROI rotation in the new Markups ROI node. It should be available in a few months.
Note that you can create optimal oriented bounding box fully automatically, using 10-15 lines of code (see New Segment Statistics: Oriented Bounding Box, Diameter and More). The script could be slightly modified to create ROI from automatically rotate the ROI to have one side parallel to a specified line.
If you don’t want to do any programming, then you can rotate the volume so that the cut axes are parallel to R/A/S axis and then you don’t need to rotate the ROI.