What do you mean by "local files... impossible with current browsers" ? Have you seen https://pmario.github.io/file-backups/ ?
Hi,
Investigate writing your own exporter using the one already there. I think tiddlywiki.com will lead you there or search this forum.
So far I can only save as registered file types but you may be able to overwrite existing files. Unless you use a file save dialogue with appropriate browser setting you will most likely be restricted to the download directory.
See how you go, I can give more info.
Regards
Tony