You can save the pages to files, either by setting the save attrtibute on the action, or setting saveresponse in the config. I don't know how you would run the pages through an online validator. I would probably use a script task (or some kind of foreach task, if you know of any) to make antcalls with some property set to each of the filenames, calling a target that used webtest to fill in that filename on the submit page and check the results. If you have a commandline version of the validator, you could probably run it with an exec task (and maybe a bit of shell script).

Another option would be to write a custom "validate" test step that would extract the text of the current page from the WebResponse (check the httpunit javadoc: method getText) and run the validation, if you don't like extra targets, script tags, and passing the page text through files.