You could attempt to alter the allowed protocols.
function wp_allowed_protocols_unc_wpse_100080($protocols) {
return $protocols + array('file');
}
add_filter('kses_allowed_protocols','wp_allowed_protocols_unc_wpse_100080');
And add links of the form file://///path/to/file.txt— see https://stackoverflow.com/questions/1369147/linking-a-unc-network-drive-on-an-html-page
I do not know if that will work. That need for five slashes could be an issue.
You could also create a shortcode.
function unc_link_wpse_100078($atts,$content) {
return '<a href="https://wordpress.stackexchange.com/questions/100080/file://///".$content.'">'.$content.'</a>';
}
add_shortcode('unc','unc_link_wpse_100078');
Add your links to post with [unc]path/to/file.txt[/unc]
That is a very simple version but it should survive the post content filters. My guess is that that option will be by far the least hassle.