When I generate the Doxygen HTML documentation for my project, I use the TAGFILES configuration option to have the generated documentation reference the documentation for some external vendor sources. The tag file generated for my sources doesn't include the functions from the external vendor sources, so doxymacs-lookup can only see the functions defined in my own source code.
It would be good if I could associate multiple tag files - those for the vendor sources plus the one for my own source code - with a single source code directory. I guess there would need to be some definition of what happens if two tag files define the same symbol, e.g. the definition from the first tag file takes preference.
I'm using doxymacs 1.7.0 at the moment (it's very nice, thanks!) but the change log entries for 1.8.0 don't seem to indicate that this has been added recently.
I might be able to find the time to make the change myself sometime if you don't have time to do it. Suggestions of good ways to do it would help in this case - from a very quick look at the code, I'm tempted to try and make doxymacs-load-tags load multiple XML files into the buffer and then have the parsers handle multiple XML files in the buffer.