I received a prompt reply from Mr. Horimoto. He recalls that Newtopia Planning was a 2nd party (his words) to Ponyca. Newtopia's boss was Koji Ichikawa, who is now boss of Locus (http://www.locus.co.jp/kaisyaannai/index.html). The development contract for Ultima - Seisha e no michi (Avatar) was given by Newtopia to Infinity through a company called Digital Presence, run by the boss of another company called Digital Arts.
So now we get a picture of the complicated make-up of the Japanese software industry, and know who Koji Ichikawa is. It might also support my theory that Newtopia wasn't a real program house.
Edit: Can't find a trace of Digital Presence, but Digital Arts is operating as a company still. Founded in 1986, made framebuffers. Now it is specialised in scan converters, though it seems to take on software development contracts as well. No mention of games in its company history.
The company history for Locus says that it started out as a software consulting company, with the localisation of Wizardry and Ultima as its primary business. I should probably mail them and ask if they're the same company as Newtopia. They were founded in 1986.