This document dates back to February January 2013, when we were doing design for V3 of the Shibboleth Identity Provider. The Shibboleth MDA (Metadata Aggregator) was also being designed around the same time, and there seemed to be an opportunity to make use of the MDA within the IdP as part of its metadata processing system. In the end, we went in a different direction but the notes are preserved here.
...
I think Chad and I saw this kind of pattern as useful because the MDA framework was designed to be extremely extensible. It's relatively easy to gin up something, for example, that blacklists any entity with entityID containing "http://iay.org.uk" (e.g., using an XPathFilteringStage). Similarly, inserting a fixed Irish flag logo into any entity whose MDRPI says its from the Irish registrar but which doesn't already have an MDUI logo defined is a pretty simple application of XSLTransformationStage.
...