Just for fun try binding MOUSE4/MOUSE8.
Edit: I see you bound MOUSE8 and it worked? MOUSE4 or MOUSE16 should do it I would think. My guess is mouse buttons work similar to modifier keys(Shift, Alt, Control). Each of these keys is assigned a value(As are all keyboard keys); Shift is 1, Control is 2, and Alt is 4. The sum of the values of each key pressed produces a single value that represents the states of each button.
0 - No button is pressed
1 - Shift
2 - Control
3 - Shift+Control
4 - Alt
5 - Shift+Alt
6 - Control+Alt
7 - Shift+Control+Alt
8 - Hankaku(IIRC)
If mouse buttons work similarly, binding MOUSE7 would actually bind something like MOUSE1+MOUSE2+MOUSE4; you would have to press those three keys simultaneously for it to work.
Edit: I see you bound MOUSE8 and it worked? MOUSE4 or MOUSE16 should do it I would think. My guess is mouse buttons work similar to modifier keys(Shift, Alt, Control). Each of these keys is assigned a value(As are all keyboard keys); Shift is 1, Control is 2, and Alt is 4. The sum of the values of each key pressed produces a single value that represents the states of each button.
0 - No button is pressed
1 - Shift
2 - Control
3 - Shift+Control
4 - Alt
5 - Shift+Alt
6 - Control+Alt
7 - Shift+Control+Alt
8 - Hankaku(IIRC)
If mouse buttons work similarly, binding MOUSE7 would actually bind something like MOUSE1+MOUSE2+MOUSE4; you would have to press those three keys simultaneously for it to work.