Display barcode: Can I use GuiXT to display the barcode for the material number in transaction MM03?

Yes; in fact one script line is sufficient:

Image (1,98)

"http://barcode.tec-it.com/barcode.ashx?code=Code128&modulewidth=fit&data=&F[Material]
&dpi=96&imagetype=gif&rotation=0&color=&bgcolor=&fontcolor=&quiet=0&qunit=mm
"

Explanation: you display an image via a URL. The URL calls up a webservice that returns the barcode for the given material number. There are several web services that return a barcode; for the above example we have chosen www.barcode.tec-it.com