There are a couple of options.
1st you have noticed why I do not like just plain /bin/cat of XML, no processesing.
Moving to Perl will certainly allow you to read one file and append (even sort) another chunk of XML. This data is also available when you preview. XML is only read and updated on form open.
You can also do true dynamic lists by using FormAPI. That can update the data anytime when the form is running. You would need a callserver to read a file off the server and a callback routine to replace the new data.