links created by anyone can already be opened in a new window if a user chooses to open it in a new window, ie right click the link and choose open in new window or in a tablet or phone hold the link down to open the dialog for a new window or tab.
It would not be advisable to treat forum content as trusted and the feature was not designed that way.
If you really want to work around it you could update all rows in mp_Users so that Trusted = 1 and you could also create a sql trigger on that table to update new rows after insert so that Trusted is always 1.
But again I advise not to do that and not to trust unknown users. It would be ok for a private site where only know users can register and sign in and post, but again, links should not open in a new window unless the user chooses that, the user should be in control of that, it is not good to force a new window on users in most cases.
Probably a better solution instead of trusting strangers you could add a little snippet of javascript (in layout.master file of the skin) to make all links in the forum open in a new window like this:
<script type="text/javascript">
$(document).ready(function () {
$(".forumthread .postbody a").attr('target','_blank');
});
</script>
or if you only want external links to open in a new window see this tutorial
But again just because you can do it doesn't mean you should, I would not do it myself because I believe users should be in control.
Hope that helps,
Joe