Am I correct to assume that it is then a just another page? If so, the interwiki takes precedence over a local page. Perhaps we can make the : character an invalid character for page titles.
There are plenty of legitimate uses for a colon in a page title. Perhaps local pages which start with something which is also an interwiki prefix should take precedence over the non-local page. Some way to link to the non-local page would be required, of course.