Xdialog has been built for dCore, for various reasons it is not available in Debian. It is a 330kb package that depends on gtk1 and glib1, making for roughly a 4.3MB SCE. A running xorg is of course needed as well. Zenity is 100MB when imported as a standalone SCE.
Since I am not fluent in fltk/c++, perhaps when things settle in base dCore and I have time I might could make the import and update routine optionally graphical with xdialog.