The tweaksets of Xara allows perfectly for changing CMS content length, the placeholders do perfectly for putting the CMS parts into a template designed with Xara. As with for example freewebshop.
Maybe some of the folks asking for a CMS forget that those CMS and shopping cart scripts are based on PHP, which as far as I know would blow the preview in Xara Web Designer.
Maybe wrong idea, maybe very off topic but IMHO Xara would be enhanced a lot if instead of applying PHP based solutions, support for PHP would be added. That would integrate debugging and running scripts into the product. There are enough solutions for PHP support. After that, all options for CMS, webshop, blog and etc. support would be logical next step.