I'm in the midst of something similar - making a Links site map available to other sites on the same server, but the only way I could see to do it was to use the pagebuilder plugin to write a clean file which could be read into pages on other sites.

I'm reading the file in via separate Perl scripts on my own server, so I can roam between virtual hosts - the problem with SSI is that it can only reference it's own domain, so you would need some external scripting to move the file(s) from one domain to another.