i’d solve your second problem like this

City
def parent; country; end
Country
def parent; continent; end

and so on!

thanks for your questions, they gave me an insight in the applications of it!!!

Is there a way to use a hierarchy-navigation method other than :parent?

What if, for example, my class has a hierarchy based on method :container?

And what if my hierarchy is polymorphic, with different bubble-up methods?
For example, a geographic hierarchy composed by [City, Country, Continent, World] where a City has its parent in a Country through method :country, a Country has its parent in Continent through method :continent, and so on?

Nice work, I enjoy it