Evan,
If the item doesn't come up in the list of available fields, it either hasn't been selected as required or isn't available for cross-reference. If the DAs aren't acting with this rule, call Support as you may have a bug on your site.
When I create a Service Request Tracker DA containing Ticket # (Item Number) field, I am not allowed to select Required for the field characteristic; but when I create another DA to cross reference the Service Requiest Tracker, I am given Ticket # as one of the fields I can cross-reference. If you want to send a login/password with Admin access to me at jfilicetti@daptiv.com and exactly what you are trying to do, I will look at your site.