GTK key theme

[Shaun McCance <shaunm wolfram com>]

I'm having problems making a GTK key theme.  I'm trying to make a
minimal set of Emacs-like cursor-movement bindings, but bound to mod2
instead of control.  I've stripped it down to one keybinding right now,
just to get it to work.  So I have

binding "gtk-super-text-entry"
{
  bind "<mod2>b" { "move-cursor" (logical-positions, -1, 0) }
}

class "GtkEntry" binding "gtk-super-text-entry"
class "GtkTextView" binding "gtk-super-text-entry"

I set the GConf value to use my bindings, but if I hit <mod2>b in an
application, I just get a "b".  If I change "<mod2>b" to "<ctrl>b" in my
gtkrc file, <ctrl>b will move left, so the theme is getting used.  On my
machine, I have the Windows keys bound to mod2 as Super_L and Super_R. 
Specifically, xmodmap returns

shift       Shift_L (0x32),  Shift_R (0x3e)
lock
control     Control_L (0x25),  Control_R (0x6d)
mod1        Alt_L (0x40),  Alt_R (0x71)
mod2        Super_L (0x73),  Super_R (0x75)
mod3        Meta_L (0x42)
mod4        Num_Lock (0x4d)
mod5        Scroll_Lock (0x4e)

I've tried it on both my work machine (RH 7.3 + Ximain Gnome2 snaps) and
my home machine (fairly stock RH 8.1 beta), and neither work.

Any ideas on why this isn't working?