Just published:
http://gimpscripts.com/2017/11/decorationthrupaths/
Filter to create a new image from the input drawable where the inward-borders created for each colour area are "painted" with a brush newly created from a set of chosen patterns.

Required:
- a set of "decorating" patterns
(default provided in the package, set of different flowers)
if you like to create/use your own, it's recommended to use patterns with transparent BG (.png)

Acknowledgments:
RobA: "Mosaic Tile Helper"
TinTran+Ofnuts: code for rotating strokes
both contributions are used inside the service function BordersInwardRepeatDTP, defined internally to the filter.

Enjoy.