well, look like it's hopeless: http://www.opengl.org/sdk/docs/man/xhtml/glReadPixels.xml glReadPixels does not work with 16 / 32 bits color.
What exactly do you mean by 16/32 bits color? Do you mean per-channel or combined RGBA values? glReadPixels works with any format that I've come across. What makes you think it won't work for your situation?
wes