[Zope] Writing custom tags "through the web"

I've found custom tags invaluable in my daily J2EE use, and would like to do the same with Zope. I'd like to be able to do it like everything else I do with zope, through the web browser interface. Unfortunately, the only documents I've found relating to custom tags tell me that I must modify the Zope source code and write these tags as filesystem products or classes. Is there no way to write custom tags through the web in Zope?