Tom, isn't the fact that you can switch off the form builders html generation by inserting your own html inside the <formtemplate></formtemplate> tags exactly what you need?
I had to deal a few times with forms that were pre-produced by designers in a way that would not be automatically producable because they were not really systematic, so I just replaced their <form> tag with <formtemplate>, and their <input foo="x" bar="y"> with <formwidget foo="x" bar="y"> and after that I was able to benefit from input validation and all other functionalities of ad_form, while maintaining exactly the look that the designers intended. That obviously has the drawback that when you do changes to the fields you also need to edit the adp file but this is of course inavoidable.
You also need to add <formerror> tags for positioning the "wrong input" error messages near the fields that caused them, and it becomes a little tricky when you want to only display certain formatting tags in case of an error (e.g. <font color="red">Wrong Input</font>), but it's certainly possible.
