Can't this be done already using the "Edit Symbol Fields" dialog in Eeschema? (related: #250)

Yeah. One by one. This tool is about mass MPN assignment by matching values.

I might be missing what you mean by mass assignment, because as far as I can tell, you can do mass assignment by matching values in the dialog I mentioned. Make sure that "Group By" is checked for exactly the fields you want to match (in your case, it looks like that would be "Value", "Footprint", and "Tolerance"), then set your MPN field for the groups you're interested in. Is this not essentially the same as what your script does?

I may also be missing something? How do I get to this dialog in KiCad 5?

Click the toolbar button in Eeschema that looks like a blue and white spreadsheet, inbetween the netlist and BOM buttons:

Then you get this dialog, which I believe is essentially a more general version of the script in this PR, as well as the others in the sch/ directory of this repository:

In kicad 5 tools -> edit fields allows to group by value. you can then change other fields of such a group all at once.
I just made an experiment in my kicad 5 under ubuntu. Activate grouping by value (marked pink in the screenshot. I also grouped by footprint in my example as i want to show that this is even more flexible. This way i can give the 0805 version a different HPN then the 0603 even if the value field is the same.) The entry at the "head" of the group can be used to update every symbol in that group.

After entering the desired value in the group level the result looks like this:

Okay @Ratfink was a bit faster than me.

Ah, nice. Didn't know that. 👍

Is there also a way to run this as a pre commit hook or is it GUI only?

I don't think it can be accessed from outside the GUI. Eeschema, unlike Pcbnew, is quite lacking in the scripting department. I suppose you may still want this script if running this as a Git hook is important to your workflow.

I'd like to defer the question of whether this should be merged or not to #250, as I (personally) don't understand why the librarians should have to maintain scripts that have nothing to do with the libraries.