First of all, Plugins can do that. FMP alone can't. Plugins are extremely conventioned, but you'll have to buy them, and the machine that runs your solutions must have them installed, and they may stop working when you upgrade to a newer version of Filemaker.
Post processing the file: your OS may have some kind of automator.
I don't know about XML/XSLT. I'd consider a field with HTML-Code.
"I need to export a text field, and retain line breaks in the output file. Any thoughts on the best way to do this?"
you can export the field contents with all its formatting and line breaks, saving it to html.
To do that, first create a calculation field ( cYourField ) with calc:
GetAsCSS ( YourField )
than create a script with only one step:
Export Field Contents [ YourTable::cYourField ; "untitled.html" ; Automatically open ]
Note that when FileMaker opens the "Specity Output File", you'll have to:
1) write into the Output File Path List --> File:untitled.html
2) check the box: Automatically open file.