mjhjmtu

I'm using xkb to create a layer on my keyboard with a navigation cluster, accessible by holding down a modifier key. Here's a simplified version of my xkb config file:

xkb_keymap 
 xkb_keycodes include "evdev+aliases(qwerty)" ;
 xkb_types include "complete" ;
 xkb_compat include "complete" ;

 xkb_symbols 

 include "pc+us+inet(evdev)"

 key <LWIN> [ Super_L, Super_L ], [ Super_L, Super_L ] ;
 key <RALT> [ Mode_switch ] ;

 key <BKSP> [ BackSpace, BackSpace ], [ Prior, Prior ] ;
 key <BKSL> [ backslash, bar ], [ Next, Next ] ;

 ;

;

So right Alt is mapped to Mode_switch, and pressing that lets me access Page Up (="Prior") and Page Down (="Next") from the backspace and backslash buttons.

These work fine on their own, but I also want to use the Super modifier (the windows button) in combination with PgUp/PgDn (that key combination is set to cycle workspaces in my window manager), and that does not work.

If I press the windows button and the actual Page Up button then it switches workspaces as desired. If I press the windows button, right Alt and backspace, that ought to be equivalent to Win-PgUp, but it doesn't seem to register anything at all.

Question:
How can I get xkb to respect the windows key/super modifer in combination with keys that I'm accessing via Mode_switch?