So I've added a feature to the ExternalArticles extension that allows a whole directory full of text files to be imported at once (namespaces are handled as subdirectories). More importantly, it also 'watches' the directories and every time a file is updated (i.e. with Ctrl-S in a text editor or IDE) it is re-imported. So this means I can have
MediaWiki:Gadget-Author.css open in PhpStorm, and just edit from there. I even have these files open inside a MediaWiki project and so autocompletion and documentation look-up works as usual for all the library code. It's even quite a speedy set-up, luckily: I haven't yet noticed having to wait at any time between saving some code, alt-tabbing to the browser, and hitting F5.
I dare say my bodged-together script has many flaws, but it's working for me for now!