Hi Plighty,
In the InfoPath 2007 Filler, you can just go to Help->Go To Next Error. qRules has GetErrorCount and ClearErrors (useful for forcing a submit), but we need to add a ListErrors. We have code that can do this and if you purchase a Support Incident we could spend time helping you use it, or you could just inject qRules and ClearErrors on submit. Or, open in 2007 Filler and find out what field is missing.
Sorry couldn't give you a silver bullet.