The XML documents I work with have the following form:
<?xml version="1.0"?>
<!DOCTYPE dmodule [
<!ENTITY TBD SYSTEM "http://myserver.mydomain.net:81/svn/s1000d/TBD.svg" NDATA SVG>
]>
<dmodule ...>
...
<graphic boardno="TBD"/>
...
</dmodule>

As you can see, the figures system location are in a subversion repository and when specified as in the sample, figures are displayed OK in Author mode, CSS associated to the documents have the following entry to handle figures:
graphic {
display: block;
content: url(unparsed-entity-uri(attr(boardno)));
}

I was trying to separate XML content from the actual subversion repository server inserting in the catalog.xml for my document type association:
...
<rewriteSystem systemIdStartString="http://s1000d/" rewritePrefix="http://myserver.mydomain.net:81/svn/s1000d/"/>
<rewriteURI uriStartString="file://" rewritePrefix="http://myserver.mydomain.net:81/svn/s1000d/"/>
...

and changing the ENTITY declaration to:
<!ENTITY TBD SYSTEM "http://s1000d/TBD.svg" NDATA SVG>

But when in Author mode, the figure is not displayed, the following error message is displayed in a red box instead of the image: "Cannot display image http://s1000d/TBD.svg"

It seems the rewrite statement in catalog.xml is not working.

Any ideas?