configuration admin files with property references in the values: how's it done?

[Benson Margulies]

I've been asked to try an experiment in moving some of the code we run
in Karaf into a minimal Felix container deployment. I've run into one
snag: Some of my .cfg files in Karaf take advantage of writing values
that contain system property references, like
${karaf.etc}/something/something.

I had a quick look around in Karaf source, but I failed to find the
implementation of this, which would appear to need to be an
alternative ConfigurationAdmin or at least Felix confadmin persistence
manager. If someone could give me a pointer, I'd be grateful.