There are various extensions that allow you to use raw HTML in a wiki page, so you can just write <a href="/wiki/..." target="_blank">...</a>. (The security implications should be carefully assessed though.)

Alternatively, if it's OK for the link to only pop up in Javascript-enabled browsers, you can wrap the wikilink in a span with a specific class and have a script run on page load and change the target attribute of links inside that class.