Lektor by Armin Ronacher is a lot more fully featured and based on a different model than most static generators. Pretty sure it handles this use case.
For example, in the past I have built a multilingual media database with Lektor, where items could be, say, books or movies, and depending on the content type I could enter a "number of pages" (int) or "duration" (time) or "author" (selection from a list of "person"-typed contents), and the admin interface would be auto-generated. I haven't seen a similar functionality in other SSGs.
Downside: For large sites rebuilding takes very, very long...