The GUI that netzen described would be pretty easily doable. Rather hard in windows, of course, as that OS is not able to loop-mount images natively, but for them, there could just be a zip with the current contents. And a binary of mkisofs.
Essentially, it would be a GUI to unpack the iso, then a frontend / caller for appbrowser to download all desired extensions to a directory named "tce" in the root of the to-be-iso, and then a caller to mkisofs with a progress bar.
Or, it would only present a graphical form of some rather easy command line tasks.
While I'm not sure I'd like to see all kinds of Windows users then coming to ask what an ISO image is, I'm not completely against such a gui either. In my own opinion it's not important enough for me to do, but if someone does, I'll support them.