Continuing the discussion from More sticky keys:
I spoke too soon.
<CapsLock> works as expected.
<NumLock> mapped to the mouse button had no effect.
<NumLock> on one of the 12 regular buttons works.
According to the HID table:
0x53 Keypad Num Lock and Clear
0x83 Keyboard Locking Num Lock
I also tried both
<keycode 83> and <keycode 131> on the mouse buttons, no NumLock.
Could this be a bug? Would someone try and see if it works for you?