Letting modifier keys act as toggles under X

Is there any way to let the Shift, Ctrl and Alt keys behave as toggles under X? I know how one could do it under CLI (for example, by doing this) but found no reference for doing so under X.

Please note that this is not a question on sticky keys, which can be enabled by an accessibility option.

From ubuntu Gödel

• Keys in X11 are remapped with xmodmap, but there is no Control_Lock in /usr/include/X11/keysymdef.h

  If you have a few keys you could map Control to something like ISO_Next_Group_Lock and define your keys in this group with "Control + Key".

  I found some info here: https://bbs.archlinux.org/viewtopic.php?id=75771

  From sroecker

• This is implemented on the DE level AFAIK. KDE has the option to Lock Sticky Keys:

  With Lock Sticky Keys enabled:

  If you press the Shift key twice then press the F key, the computer interprets this as Shift+F. Now if you type a P, the computer interprets this as the letter P (Shift+P). To de-select the Shift key, press it again.

  http://docs.kde.org/stable/en/kdebase-workspace/kcontrol/kcmaccess/index.html

  Gödel : That's a good answer for KDE users, though it isn't for GNOME users like myself.
  From scottl