What you can do is to create an ISO file. You can either copy the ISO file on a dual layer disc or split it using WinRar and copy the files to multiple single layer discs. Send the disc(s) to your replicator and they will be able to create the DLT tape from the ISO file.
We have been using a company called New Cyberian Systems for all our DVD replication. Besides providing good prices they are the only company that's willing to experiment different approaches. We had the same problem you are facing now and New Cyberian helped us solve it beautifully. They website is
http://www.newcyberian.com.
Good luck!
Audrey
