I would be careful about using zisha over direct heat - most clays used for kettles are chosen because of their ability to cope with thermal shock, which allows them to be used over gas, charcoal, etc. Zisha, to my knowledge, isn't very good for this and may crack (maybe not the first use, but over time as the clay body is repeatedly stressed).
Global Tea Hut just commissioned zisha kettles and they recommend only using them over an alcohol flame, as it's a more gentle heat. However, this means heating the water in a different vessel before transferring it to the zisha kettle. I might follow the same guidelines with the pot you're talking about.