One way to see the unpublished files show up in the search results is by hacking one line of one core file, in wp v. 3.5.2 on line 764 of "\wp-includes\class-wp-editor.php", comment out this line:
'post_status' => 'publish',
So commented out, it would be like this:
// 'post_status' => 'publish',
Of course hacking core file means that next time you upgrade your hack will be lost and you'd have to go in and make the same hack again so that is only a solution if you want to live with that headache.
Looking into it further, searching found a feature request that pointed to an enhancement request, which sounds exactly like this request: Make WordPress Core ticket #18042
On that page, one person commented that:
"Definitely +1 for a hook or pluggable. Need a way to add items other than just published posts to the list of available items to link to."
On that page, another person linked to their plugin Better Internal Link Search
Going to that plugin's page, one of the features listed is that:
"Scheduled posts are included in search results."
From the comments, looks like over the last few months some people submitted a patch for this, but it hasn't yet made it into the 3.6 core, but is scheduled for some future release after that.
If one logs in to the core.trac.wordpress.org site, one can scroll to the bottom of the ticket and turn on the "Add to Cc:" checkbox for oneself, to be added to the mailing list to be mailed whenever the enhancement request status changes.