I have a Lenovo N700, which appears as both a mouse and keyboard (connected via bluetooth or wireless, all details here are while its connected as bluetooth). Currently it has id 14 as seen below.

⎜   ↳ Lenovo Mice N700                          id=14   [slave  pointer  (2)]
        Reporting 8 classes:
                Class originated from: 14. Type: XIButtonClass
                Buttons supported: 13
                Button labels: "Button Left" "Button Middle" "Button Right" "Button Wheel Up" "Button Wheel Down" "Button Horiz Wheel Left" "Button Horiz Wheel Right" "Button Side" "Button Extra" "Button Unknown" "Button Unknown" "Button Unknown" "Button Unknown"
                Button state:
                Class originated from: 14. Type: XIKeyClass
                Keycodes supported: 248

I wanted to remap one of the letters (the letter c) to some other key (in testing, I just use 'q'). The reason being a side-swipe on the touch-sensitive pad on this mouse would produce c-down, Super-down, c-up, Super-up (I have evemu-record output if that would be useful). Which would basically type the letter c wherever my cursor happened to be, which is annoying (for example in gmail's web interface I'd get a compose window).

When I get the current keyboard map using xkbcomp -i 14 :0 map.xkb, I can edit it to change the output. I then try to apply this output using xkbcomp -i 14 synch map.xkb :0

This does not change what the key does on my mouse, even though re-running xkbcomp -i 14 :0 map.xkb shows the change did 'take'. So I tried various inputs out of the below:-
⎡ Virtual core pointer                          id=2    [master pointer  (3)]
⎜   ↳ Virtual core XTEST pointer                id=4    [slave  pointer  (2)]
⎜   ↳ ETPS/2 Elantech Touchpad                  id=13   [slave  pointer  (2)]
⎜   ↳ Lenovo Mice N700                          id=14   [slave  pointer  (2)]
⎣ Virtual core keyboard                         id=3    [master keyboard (2)]
    ↳ Virtual core XTEST keyboard               id=5    [slave  keyboard (3)]
    ↳ Power Button                              id=6    [slave  keyboard (3)]
    ↳ Video Bus                                 id=7    [slave  keyboard (3)]
    ↳ Video Bus                                 id=8    [slave  keyboard (3)]
    ↳ Sleep Button                              id=9    [slave  keyboard (3)]
    ↳ ASUS USB2.0 Webcam                        id=10   [slave  keyboard (3)]
    ↳ Asus WMI hotkeys                          id=11   [slave  keyboard (3)]
    ↳ AT Translated Set 2 keyboard              id=12   [slave  keyboard (3)]

A summary of what I've found:-
Using input 14 does not change anything
Using input 12 has two different results:-
   If I start typing c on my keyboard the letter c comes out. However if I side-swipe on my mouse, the letter c comes out, if I then switch back to keyboard, thereafter the letter q comes out if I use mouse or keyboard
   If I start by sideswiping with my mouse the letter q comes out, then the letter q will always come out whether using mouse or keyboard
   My conclusion from using input 12 is that using the mouse somehow 'activates' the xkb change for both mouse and keyboard
Using input 5 does not change anything
Using input 3 has two different results:-
   If I start typing c on my keyboard, the letter q comes out. Side-swiping then gives q, but switching back to the keyboard gives c from then on
   If I start sideswiping on my mouse, the letter c comes out, and the same from the keyboard thereafter

My summary so far has been that the usage of this mouse somehow resets input 3 and/or 12. This may have something to do with udev etc., I am not familiar enough to know. I'm pretty sure I've found a bug, but have no idea what the bug actually is.

For reference, I'm running Arch Linux 64-bit, so pretty much the latest release of everything.