I'm afraid that plain X will not allow you to do this. You may want
to consider something small and light like ratpoison
first doesn't have window decorations to be begin with as it's intended
to be driven w/o a mouse (hence the name). Fluxbox can be easily modified
to not display a tool-bar, and you can take away the window decorations so
people only see a minimal frame around your apps. Both these window managers
have a marginally small memory footprint.