I remember using a browser extension for this a long time ago (it was inserting something like an “open in external editor” button next to each textarea and saving in the editor was updating the textarea content). I could not recommend a specific one now, but there seems to exist a few of those on Search results for "external editor" – Add-ons for Firefox (en-US) (I guess it’s true for Chrome too) for example.
You might want to keep an eye on https://extensions.xwiki.org/xwiki/bin/view/Extension/cli which aim at making it easier to edit any part of XWiki with any editor (it’s not very active, but I know some people are using it and want to improve it).