If you haven't already done so, please download a free trial copy of FileMaker so that you can test things out for yourself. That's the best way to get a comprehensive understanding of what FileMaker can and can't do for you. And we'll be happy to answer any questions you have here while you evaluate the application.
There's a function in FileMaker known as GetAsCSS. You can define a calculation field with an expression such as:
GetAsCSS ( YourTextField )
and then you can export this calculation field in place of YourTextField to export the data with CSS tags that will include the HTML text format tags you likely need in order to export both the text and it's format.
Thanks PMJ, that sounds like the answer. I have bought FMPA as it seems pretty clear it'll do everything I need.