I use an old freeware program called Open Site Editor which uses ASP script to create an editable (textarea) page. This is basically a blank page that is set up to be editable using the <!--BEGIN EDITABLE-->
(this is where your editable content goes)
<!--END EDITABLE--> tags.
This page exists on your server and is independent of the XWD pages that are uploaded. Then I use the placeholder function of XWD to insert this page into the XWD page. It works very well for me. If you don't have asp capabilities, there are other freeware programs that can do the same thing. The key is to create the content page as a separate page and insert using a placeholder. When publishing your page (frontpage, dreamweaver) make sure that you have the content page set to "not publish" (after you have everything set up) otherwise you will overwrite you client's current page with your initial set up page.