Finder: keep sort-by per-directory Some directories make sense to be sorted by date modified, while others make more sense to be kept sorted alphabetically. 

How can I make it so that finder will apply a specified sort by selection on a per-directory basis? 

If no such feature exists (In which case I hope somebody at APPL is paying attention), I suppose using/remapping the keyboard shortcuts (Defaults being Cmd+Ctrl+5 for Modified and Cmd+Ctrl+1 for Alphabetical by Name) is the next best thing.
Finder should already remember both arrange by and sort by modes for all normal folders. But in column view and list view the current folder is the root folder of the window. Or if you use column view, it is usually some location in the sidebar or the computer view.

I don't think there is any way to change that, but if you open folders by selecting them from the sidebar (or for example from Alfred), you could add folders to the sidebar.

I'd actually like the opposite and make all folders default to arranging by none and sorting by name. Deleting .DS_Store files only does that temporarily until you change the sort by or arrange by modes manually.