You can use the new Office Web Apps 2013 Server not only for opening/viewing documents from SharePoint 2013 but also to open files directly from a URL or a file share.
The link has to look like the following:

http://webappsserver/op/view.aspx?src=

If you do not want to build the path manually, there is a web page that does the generation of the URL or the -Code for you:

http://webappsserver/hosting -> redirects to: http://webappsserver/op/generate.aspx

Important: Make sure that the machine account of the web apps machine has the appropriate rights on the file share or URL!

Addendum You need to have “OpenFromUrlEnabled” set on your OWA farm.

Markus