I just noted Which per-site-metas are offered in the chat-room creation dialogue? (pointed out by Shadow Wizard). The conclusion of the experiment there was:
This experiment suggests that only the meta sites, for which at least one room already exists, are offered.
...the distinction between beta sites and graduated sites is not sufficient to explain this.
So, if I'm reading it correctly, it is currently impossible to create a room hosted on those child metas which do not already have an existing meta room (smells like a bug!)? Or is there a way to bypass this restriction somehow?
I can think of several legitimate reasons as to why a site may choose to keep their main chat rooms and meta chat rooms separate. Killing off that freedom of choice, and that too non-uniformly, isn't a good idea. If this really is a bug rather than status-bydesign (for a good cause), then it should be fixed!