Thanks for helping look into this Jim.
GetSaveFile probably accepts the / as well. (I didn't test it).
I have always been a little disappointed in UI.openpanel.
Using Win32API.so (which requires that you install a second file with your plugin and only works with Windows Apps), lets me specify patterns for acceptable extensions, and also to specify a default extension to be appended if the user doesn't enter one.
And the GetOpenFile version lets me require that the file exist before the use can hit OK.
Small things. But it is too bad that SketchUp can't find the time to update some of their routines. (This could easily be done with either new routines, or optional parameters, to make them a little more useful.)
However, the main point of the original post is that it would be nice if you could pass write_default() a string and have it save it and return it, without having to "reverse engineer" which characters don't work.