As far as I understand I just have to extend org.jboss.seam.pdf.ui.UIRectangle or even org.jboss.seam.pdf.ui.UIImage.
I never used iText & Barcodes but if you can convert your barcode to com.lowagie.text.Element you should be able to show it with p:element (have a look at http://www.jboss.com/index.html?module=bb&op=viewtopic&t=105900)
I have a working basic implementation of the tag if anyone is interested, it supports absolute positioning/rotation. It has all the functionallity I need now but I plan to extend to most of the underlaying implementation.
Please to open a JIRA case and contribute the code back. I can see how barcode generation might be useful to a lot of people.
I created a case and uploaded the files, note that I only need code128 type of barcode at this moment, so I didn't test much the other types. There is also an example of use in the package.
The functionality for code128 is also not complete noe but i expect to expad it in the following days.
It supports absolute positioning and rotation to be able to place the barcode outside the document area
<p:barCode type="code128" code="absolute" xpos="200" ypos="300" rotDegrees="45"/>
coordinates are in points from base-left of the page, rotation is in degrees.