Thanks for checking back with me! And no, I still haven't sorted this one out, ha. And I am indeed using the PageForms extension for all of this, btw.
To start, I have one category 'Star' which currently only contains two entries, 'Sol' and 'Alpha Centauri'. Each is a page and a category of the same name, which is then used to hold the pages and categories for subsequent planets.
For example: The page 'Sol' is in Category:Stars and Category:Sol. The page 'Earth' is in Category:Planets, Category:Sol, and Category:Earth. The page 'Luna' will be in Category:Moons, Category:Earth, and Category:Luna.
In a slightly different example, the page 'Alpha Centauri' is in Category:Stars and Category:Alpha Centauri. The page 'New Terra' is in Category:Planets, Category:Alpha Centauri, and Category:New Terra. The page 'Luna 2' is in Category:Moons, Category:New Terra, and Category:Luna 2.
And so on and so forth.
When creating new locations, I select a Star and then a Planet, but since the Planet selection only pulls from Category:Planets, it shows all planets in all star systems. So the second dropdown uses 'values from category=Planets' but I would like to somehow cross-reference with a second 'values from Category=STARNAME' so that it only pulls pages which match both categories.
The final kicker for all of this is that I will have one final form for submitting new locations, which will need to be drilled down through Star>Planet>Moon. However, there is a 'None' option for moons, so in the case this is selected the form will need to recognize that the new page needs to be in Category:PLANETNAME, and not in Category:None (for the moon selection).