Since the approach you want varies radically from the way
Struts normally handles validation, I'd recommend that you abandon the use of the struts validation framework and write your own validation in javascript and possibly AJAX.
If your validation can all be done on the client side (simple validations such as "is not blank" or "is integer") you can do it with just javaScript. If it is more complex (valid account number, etc.) use AJAX, which allows you to call server-side functions from javaScript. I've used an implementation of AJAX called DWR, and found it fairly easy to set up and use. Here is the link to download it:
https://dwr.dev.java.net/ Basically, when you sense an error, you use the window.open() function of javascript to open a new window with a specified size and location. You can then format the window any way you want.
If you want a modal dialog box, Microsoft Internet Explorer has a showModalDialog() method in its version of javaScript that allows you to open a modal dialog box that is formatted however you want. I'm not sure if the other browsers have an equivalent function or not.
[ January 05, 2006: Message edited by: Merrill Higginson ]