Sorry for the late reply, I must have missed this somehow. Yes, there is an option when you bundle a Z-machine game with Ozmoo on the C64 to add a loading screen, so rendering of the graphics actually happens while the interpreter and the story file is being loaded. In other words it is not part of the Z-machine and is displayed before the Z-machine takes over.
Yes, the Hibernated sequels will be written in PunyInform. I don't think I'll give The Curse of Rabenstein an Inform treatment, but I recently laid out some ideas for a Rabenstein sequel that definitely will be made with Puny. And I have, very recently finished a new game "The Ghosts of Blackwood Manor" (going to be released soon) which is written in Infocom's advanced Z-machine format (Z5), also with Puny as a library.