I’m reminded that Plone lets you create validators at the field level, ie. a string field that should contain an email address, as well as at the complete form level, ie. checking that if one field has a certain value in it sets constraints on the values that can be in another field on the same form.
In a similar vein, perhaps this content quality plugin framework project could process not just the current item but could allow a plugin to specify other content on the site to process. An example might be to check for broken links on the entire site, versus just checking for broken links on the current page.