Yes it is possible, look into the seam examples, you will find an example called seam-itext. In this one, there is a JFreeChart embedded in a html richface panel. the name of the xhtml file is chartimage.xhtml.
Yes, please look at the example app for HTML usage. The original use case of the tags was to generate charts for PDF reports. The HTML support came later, but unfortunately the tags share a lot of code with the PDF versions. I'd love to detangle them some day. :)
Somehow I just get the impression that it is to be displayed to a PDF because it uses the p namespace. I'm going to have a look at the examples.