MRX37, your comment has really brought up some interesting and informative responses.
Putting my own humble opinions in response to your thread and the following responses, if all the engineering data for building a 15GP22 still exists in a RCA archive and there's a rich CT-100 hobbyist out there that is a millionaire and is willing to spend astronomical amounts of money in rebuilding a mini 15GP22 tube plant built to specs matching that of the original plant or a modernised plant with more refined modern tube making machines that do the same job, and of course the materials needed for the CRT then I would dare say making new 15GP22 tubes would be possible.
However it's obviously not a profitable project as the majority of people in USA are not interested in owning a colour TV set from the 50s let alone wanting new tubes, they want a wide plasma screen TV.
So to make new 15GP22s is weighted on being able to obtain all the engineering data for the CRT and the CRT building plant equipment and someone who is willing to spend astronomical amounts of cash in making a CRT plant to do the job and the materials.
I can somewhat see the possibility of some major electronics company remaking CT-100s utilising modern electronics and modern round tube CRTs made to modern CRT specs as a retrospective release by demand if enough people in USA are interested in owning a classic style RCA CT-100. This sort of project was successfully done with the classic 1958 Philco Predicta TV set, Telstar are making these sets with colour CRTs and making them in a range of styles and colours, check out the site
http://www.predicta.com/index.shtml .
So I go with the flow and say rebuilding the old 15GP22 is the way to go.