I've done some investigation of what's needed to get aq source CD for debian-edu that contains the sources for everything on the binary CD. In fact debian-cd does the best job it can at this; the source CD contains no extra sources for packages not on the binary CD. It is missing sources for the packages listed below, so we just cannot fit all the sources on one CD after all. Everything does fit if we make two source CDs.
atftp atk1.0 autoconf bash (2.05b-2-24) courier (0.47-3) curl (7.12.3-2) cvs ddd enscript espa-nol gcc-3.4 (3.4.3-6) gcc-defaults (1.19) glib1.2 gnupg-doc gpgme1.0 gtk+1.2 gtk+2.0 gv ifplugd ifrench igerman98 imagemagick ispellcat j2re1.4 java-common kde-i18n kdeadmin kdeartwork kdegraphics kdemultimedia kdenetwork kdepim kdeutils koffice koffice-i18n latex-xft-fonts lesstif1-1 libdaemon libgsf libraw1394 menu mozilla mozilla-locale-ca mozilla-locale-de-at mozilla-locale-fr mozilla-locale-hu mozilla-locale-ja mozilla-locale-ko mozilla-locale-no-nb mozilla-locale-pl mozilla-locale-ptbr mozilla-locale-sl myspell (1:3.0+pre3.1-10) neon0.23 norwegian openoffice.org openoffice.org-debian-files openoffice.org-dictionaries openoffice.org-help-en openoffice.org-help-es openoffice.org-help-fr pango1.0 pgpgpg poster psutils rdesktop stlport4.6 t1lib unzip unzip-crypt util-linux (2.12-10) wv2 xmms zip zip-crypt -- see shy jo

