It would be nice if Include files were subject to the same variable interpolation as regular properties.
e.g. it should be possible to do
include=${sys:myfilename}
I think it is simply a matter of changing the call in to loadIncludeFile() PropertiesConfiguration.propertyLoaded() from:
loadIncludeFile(files[i].trim());
to:
loadIncludeFile(interpolate(files[i].trim()));
I am happy to submit a patch.