DOCBOOK-APPS: External Graphic references?

["M. Wroth" <mark@astrid.upland.ca.us>]

I'm not sure this is really a DocBook question, but I'm encountering it
in a DocBook extension :-)

Is there anyway to specify the location of an external-graphic in
something like a catalog file?  My extension includes a logo file in
the DSSSL stylesheet (which one being controlled by a "class"
attribute in the main document element).  At the moment, the only
way I seem to be able to tell the stylesheet where the relevant file is
located is with an explicit path in the DSSSL code -- which is not a very
portable solution.

What I would like to do is to specify the location of the logo
file with a formal public identifier and fix the location of the system
entity in the catalog file.  But if that's doable, I don't see how;
external-graphic sakes a system identifier, not an FPI.

Mark