You lost me slightly there - so if the WP site is set up to generate URLs of the type
http://sitename.com/?page_id=123
you could set up a .htaccess rule that would map a non-existing page to this external page? I presume that doesn't apply the theme to the page then?
I'm asking this for 2 reasons:
1 - I have a friend who is writing some PHP code which I would love to integrate in a page on my site, and as yet I have absolutely no idea how to do this (but having at least the page accessible would be cool)
2 - I discovered that it was possible to access the raw site 404 page by feeding it a URL in the style of http://sitename.com/blabla.
Adding a statement to the root .htaccess forced that too into the WP site (and theme):
ErrorDocument 404 /?page_id=999999
I presume yours is similar (not a .htaccess specialist myself)?