Is there a package that will let me specify the full path of a file that hasn’t been saved yet? I know I can use the AdvancedNewFile package to specify the full path where a file should be saved, but I would like to do the same for a file that has already been created (but not yet saved).

May be, that I understand you wrong.
But if you create a new file in ST (a new buffer) without saving, this file not really exists. It has no name, no scope and so a lot of plugins doesn’t work because they need a scope.
So i don’t see the sense behind this.

Specify where? And what for?

It’s stored in the Sublime session, which is in the app folder (OS dependant) and cannot be changed. The portable version probably puts the session file in the install folder though.

@FichteFoll. This is a common use case:

1. create new file
2. start typing
3. realize that I want to save the file in a specific location: “/my/specific/path/my.file”
   4a. pull up the save dialog, where I can’t just paste the path because it’s not in the current directory
   4b. copy text, pull up ANF: New File, paste the path, paste the text in this new file
   4c. a better alternative?

AdvancedNewFile should help.

Well, I can say that the Windows “save” dialog allows entering absolute paths. Other than that I only know that it is at least possible to change where a view points to (i.e. which file) on the API side.

this works:
superuser.com/a/1021414/295075