John will write such a converter. If this is allowed in a PI, then the PI could be overridden. Maybe the processor should say which set of catalogs should be used. Often they're used by the recipient to overcome deficiencies in what the author set. The author can also use them, by using the catalog with a set of entities.
Packaging requires a catalog.
We aren't doing packaging. We really couldn't do more than a hint of where a useful catalog might be. John will write up a trial. Everyone supports this proposal, even Paul after lots of discussion. A few issues now fall off, such as the question about a stylesheet entry.