Question: Is there a way to have a RewriteRule fire on any domain that is requested on a box? For example, I have 5 VirtualHosts that would have the exact same RewriteRule applied to them and currently each VirtualHost directive has the rule, but can I put this somewhere 'global' so that upkeep is easier?

Answer: 
No, I'm afraid not
You can, but it is somewhat of a hack.
Create your RewriteRule in a .htaccess somewhere like:

> 
> /www/redirects/global/.htaccess
> 
> 
> 

Now just set the DocumentRoot for your virtualhosts to /www/redirects/global/ and every request will be forced through your RewriteRule's.