I'm still not satisfied with that. The only problem prohibiting it to be run individually from Eclipse's or IDEA's testng integration that I see is that SeleniumTestCase needs to be provided with a web app folder. This normally comes from the testng.xml.
I propose adding an annotation @WebAppFolder to tapestry-test with which developers can specify the web app folder in which this test class should run on a per-class basis. SeleniumTestCase will be changed to look for the presence of this annotation and if present, use the value provided there. Otherwise it would take the value from the xml config file or fall back to the default of src/test/webapp.