I could not get this to work for me.
Could also not deal with button mapper app using a long press for volume down, as it activated the app but had to wait 30 seconds for voice access to time out. It could also activate in pocket or not when really needed.
What worked for me was some of the steps on the neutron wiki.
I am by no means anywhere near as knowledgeable as others.
The section proved helpful [Login to see the link]
Once I did all the steps there, I could then edit the mtk-kpd.kl file. I copied out his "module.props" and edited that to my liking.
I changed the name of "QUICK_DIAL" to something else. Note - don't change the volume or power keys, as noted in the link.
After rebooting, I was able to use the key on the back of the phone (or front, I guess, if it's closed). In the voice access app, it was labeled "Camera" but it worked!
I could not get the "MESSENGER" key to remap correctly. It seemed to break the 2 shortcut keys as well as the 'send' key. Not sure why. edit: changing it to say FOCUS worked.
Greatly prefer that key as it is not an external key that can be bumped. I do not know yet how to make this into a package for easier install for others, but may try to figure it out.
Hopefully this helps others who want to try remapping!