I didn’t want to put this as a ‘Feature Request’ cause it’s too undercooked. Plus knowing Martin’s methodology (and NOT knowing how this place is actually contracted back end) I wanted to post this “talk through” of an idea I believe should be easy to implement (in some way, if not “my” way as such).
I just can’t imagine what the easiest or most logical way of doing it is.
Here is two versions (with different pros and cons) which I can see it operate.
My (pretend hypothetical) story chapter is called “Adventure of Gary: He turns into a banana”
Idea 1
In the story title box I actually type something like:
“[Adventure of Gary: /Navhide] He turns into a banana”
or
“Adventure of Gary: [N:He turns into a banana]”
And the result is, when the story appears on it’s page or anywhere else on site,
it appears as
‘Adventure of Gary: He turns into a banana’
But, when the story chapter appears in the chapter navigation bar, you see
‘He turns into a banana’
(not ‘Adventure of Gary: He turns into a banana’)
Pros: you will only be using it if you know how to use it, and it won’t effect users naive or, how the site functions in any way.
Cons: ehh… error prone people ( 
) will have to trial and error to make sure the title appears as they intend in both formats (navigation and normal)
Idea 2
The other way of doing this (different pros, different cons)
Is adding (yet another) box in the submission window, where you can give each chapter, a special shortened Navigation button title.
Pros: very idiot proof
Cons; mistitling or misuse would create less, but more chaotic mistakes, also, yet another submission page input box.