In particular, I don't think you can make a share-alike requirement on a public domain item.

That's correct.

"Public domain" is sometimes used in a much vaguer sense to mean the information is out there and being used and shared, but this vaguer sense is best avoided.

I suggest to Epiphany to either: * Strike out "public domain" and replace it the idea of being "open licensed", or * As Alicorn suggests, declare it public domain. Creative Commons has tools for this (the advantage being that you give a lot more clarity - so I know that you mean the same thing as I understand by public domain). See the nice summary and: Apply CC0 to your own work.

Hope that's helpful.