Your hunch is right, SlicerRT has this function. It’s not that trivial, so it’s good you asked. Currently there is no built-in conversion to planar contours in Segmentations, but it is possible to do via DICOM RTSS export. Make sure you have a valid patient and study in the Data module, and have the master volume and your segmentation under the study. Right-click study, choose Export to DICOM, then make sure RT is selected as export type. The image will be re-exported (see details here), but you will have an RTSS series. Load it in Slicer, and then you’ll have the planar contours. Let me know how it goes.