There really is no reason to back up an .ost file as it is, by design, an offline version of the data that is on Exchange and will be synchronized when you are connected with the server. It is, in effect, a temp file, to read your data when not connected with the server.
The only workaround that I can see is to periodically zip the .ost file into an archive, which will be backed up if you select the file type "other" and locate it in a folder that is scanned for new/changed files to back up.
-steve