I am using Tkinter with Python 2.6 and 2.7 for programming graphic user interfaces.
These User Interfaces contain dialogs for
call method is defined in _tkinter.c, but there is nothing interesting for your particular task there. It just calls a Tcl command, and the command tk_getSaveFile does all the work.
And yes, when there is a native file dialog on the operating system, tk_getSaveFile uses them (e.g. GetSaveFileName is used on Windows). It could be possible to add widgets there, but not without tampering with C sources of Tk. If you're sure that your target uses non-native Tk dialogs, you could add something to its widget hierarchy by hacking ::tk::dialog::file:: procedure from Tk (see library/tkfbox.tcl).
I would rather take an alternative implementation of tk_getSaveFile, written in pure Tcl/Tk and never using the OS facility. This way, we can be sure that its layout is the same for all OSes, and it won't suddenly change with a new version of Tk. It's still far from trivial to provide a convenient API for python around it, but at least, it is possible.