I have some artwork made in Inkscape, which I would like to export as a 9-patch image (info here). Does anyone know how I would go about doing this? If this functionality isn't supported out of the box, is there any plugin that might achieve this?
9-patch images are not a real file format but only a convention. Export your image as a png and you are done, assuming that you already drew the 1px border around the actual image. If you want you can rename your file
image.9.png to emphasize, that the image is used as a 9-patch.
If you just have the actual image, expand the canvas by 1px on each side and draw a black 1px wide rectangle on the top and left border. The black lines define the stretchable area.