What do you mean by "getting the wiki to see an external RSS source"?
Do you mean that external RSS readers can't see your wiki's feed? MediaWiki
doesn't read external sources... but if you want an external feed reader
(e.g. Google or Yahoo) to see your wiki, then you need an intermediate page
that can be seen outside your intranet BUT can also read your intranet.
What we did is create a PUSH script that sends the RSS text to a server that
sits in a DMZ. That server then has a URL that external feed readers can use
to retrieve that text. The server in the DMZ can't read the RSS directly
because of the DMZ - although we could certainly open the firewall to allow
it. However, we decided it was far safer to prevent a server in the DMZ from
accessing the intranet, although it is open the other way around (intranet
servers can access the DMZ server).