gimp-ora-plus exports a GIMP xcf file as an OpenRaster file. GIMP already has OpenRaster export built-in, but this plugin will also export your paths, which the built-in functionality does do. Additionally, this plugin will let you associate arbitrary attributes to your layers and paths.
To install, put gimp_ora_plus.py in your plugins directory.
To export, open up GIMP and select File -> Export as OpenRaster Plus from the menu.
To specify custom metadata on a layer or path, you'll need to add to add a url querystring-like string to your layer or path's name. For example, if you name your layer:
rock?type=obstacle&weight=50
The resulting xml metadata will look like:
<layer name="rock" type="obstacle" weight="50" />
Which you can then use in whatever other program you're importing this data into.
Specifying an attribute twice like this:
wall?material=wood&material=metal
will create a comma-separated array like this:
<layer name="wall" material="wood,metal" />
Paths are exported as CSVs, following the path-csv format.