Since the folks at Artisitic Nippon seemed to know their stuff, I sent them email asking them to tell me more about yokode kyusu. Toru Yoshikawa was very helpful and wrote back in just a few hours. Because I prefer to get my information from more than one source, and because Yoshikawa-san also stated in his email that his "comments are not Academically proved," be aware that the following information may not be accurate. However, it is all I could find, and it seems legit, so I am going with it.
"Senchado is similar to Chado - the matcha tea ceremony. However one of the differences is that in Senchado metal kettles are not used boil water because it is believed that it affects the taste of the sencha in a negative way. Instead, a terracotta pot called "bofura" is used.
Sencha was introduced in mid 17th century to Japan from China, and this bofura originally came from China too. It has a handle on the side and it is therefore considered to be the fore-runner of the yokode kyusu. In the early Onkoyaki kiln workshop (Gifu-pref) established in 1859, houbin (teapots without a handle) and yokode kyusu were produced. Old banko kyusu were also in the yokode style. Onkoyaki no longer exists, but the area of Onko is in the same vicinity as Tokoname and Banko and therefore we can assume that yokode teapots were produced by these kilns at around the same time."