Under Windows it is possible to convert TiddlyWiki into a true local application by renaming the HTML file to have the extension *.hta. The fsosaver module can then use the ActiveX FileSystemObject to save changes.
Note that one disadvantage of this approach is that the TiddlyWiki file is saved in UTF-16 format, making it up to twice as large as it would be with the usual UTF-8 encoding. However, opening and saving the file via another saving method will re-encode the file to UTF-8.
See Wikipedia for more details: http://en.wikipedia.org/wiki/HTML_Application