I use Chrome to save without any extra addons. While it's not the most ideal workflow, it's completely generic and 100% portable (which, you noted, is not that important for your use-case). Wherever my TW document is stored (either local or hosted online), I can edit it and, when I press the save button, I get the exact same "download saver" interaction. One thing I've done to make it a bit smoother, is to set Chrome to "always ask" for the download location. It defaults to the same folder as the current TW file, so when it shows the folder list in the 'save file' dialog, I just double-click on the current document's filename, and then confirm "ok to replace existing file". I then can then press F5 (refresh) to reload the document.
-e