First off, if we did use the alternate structure, "Planet of the Apes" will become "The Apes' Planet" and "Ape Planet," but not "Apes Planet."
In "Ape Planet," the first Noun is already acting as an Adjective so it can't be Plural. But its meaning is Plural.
I know the direction your question is going but your choice of examples isn't so good. Everybody knows "Planet of the Apes" as the title of a novel later adapted into TV and film. It works better too with that as a title (better than "The Apes' Planet").
For your second example, we sometimes have a lot of Nouns as Adjectives. For example, "data traffic reduction scheme."
Newspapers usually do this to their headlines. Because they need to save space. But I personally find them harder to read:
Data Traffic Reduction Bill Passed!
So, your example "extra work proposal," you could write it down like that and I'd just read it backwards.
You can also just write it clearer as: "Proposal for Extra Work."