If you use the latest JTidy snapshot then it usually works around the problem:
Note that Seam support is not done yet and was not included with Beta 1. You can build JSFUnit yourself and use the SeamClient which extends JSFUnitClientSession. That will allow you access to conversation-scoped managed beans, though you currently will get a harmless stack track with each access.
Sorry for the jar hell. It's also on the todo list to provide better tools that automatically turn your project into a "JSFUnified" project. If anyone in the community wants to work on that, I'd love to have some help.
Indeed, using jtidy-8.0-20060801.131059-3.jar resolved the issue. Thanks!