While we only directly support questions about the official spins, the following occurs to me:
- It's perfectly plausible that a person turns up with what they think is an Ubuntu problem but the best solution requires another distribution.
- It's arrogant to assume Ubuntu will always be best for every scenario.
- It's self-defeating to limit the site to answers that only mention Ubuntu-related things because that would —in these circumstances— mean we're excluding the best answer.
That said, if the question is "which version of Ubuntu will fit on a CD?", "try Puppy" on its own is not a valid answer. It doesn't answer the question. But you could answer the Ubuntu question and then go on to add your recommendation. That only makes it a better answer in my eyes.
So in this case, if you were sure there are no Live CD versions of Ubuntu, you could say:
There are no supported Live versions of Ubuntu that will fit on a CD.
However there are other Live distributions of Linux that still target CDs and smaller. My favourite is... because... etc...
But that does also rely on you being technically correct. Which may not be the case. Lubuntu 14.04 is less than 700MB. Mentioning all of this in a structured and well-reasoned way would probably make for a pretty decent answer.