It is fairly simple to add multi-user support to your Android 4.2 rom using the device overlay.
To enable multi-user support you will have to add the following to your config.xml:
This will enable the Users section in Settings to add/remove new users.
Additionally you will have to modify layout-port/keyguard_host_view.xml, because the layout-port version of keyguard_host_view.xml does not have the multi-user selector.
You will need to add the following to the layout:
Instead of modifying the layout xml directly you can also just add a modified version of the .xml to your device overlay.
That will add the user selector to the keyguard/lockscreen.
The changes can be found here:
In case there is just one user it will not show the user selector in the keyguard/lockscreen.
* The dialer only works in the main user and and trying to dial in any other user will result in a crash, because the dialer does not support multi-user yet.