Wonderful, thank you @saxoncameron.
I actually wanted @seanblue’s “Show Specific SRS Level” script, but for whatever reason, it doesn’t work for me on Google Chrome (script has no noticeable effect).
This one does work, however. So I’ve just tweaked my local copy of the script to replace @saxoncameron’s rank names with Apprentice 0 through 4 and Guru 1 and 2.
@The1AndMany’s last comment was helpful here, too, as without it, it may have taken me a while to realise that I needed to remove the spaces in the rank names used under the hood. Though I don’t know where it is that The1 has encountered said bug, or where this rank name “Mostly Harmless” came from? Did you also replace the rank names with space-containing variants, The1?
Happy to help out anyone wishing to emulate this, but basically: You can just replace the names in the SRS_RANKS list initialised in lines 25 and 26 with your preferred names (applicable line numbers are likely to change with future versions of the script, of course); and if your preferred names don’t include any spaces, then you’re done, but if they do, then you can do what @The1AndMany did. 
Unless, of course, there is some problem with this solution that I am still blissfully unaware of.
I guess the ideal thing would be to provide a “settings” page where users can set the rank names to whatever they want.
Cheers