I have a custom X11 keyboard layout that I can use with setxkbmap. But after some searching I've found that apparently just having the XKB layout file isn't enough to set it as a new keyboard layout in KDE 4. What I would like to have my custom layout be recognized by the KDE system so that I can choose it using the systray applet, or install it as the default using the System Settings module. What do I need to do to make this happen? If it's complicated, that's fine, I would still like to know what is involved.I've looked at this question which seemed to be related, but it apparently is about creating the keyboard layout in the first place, which I've already done.
How can I install a new keyboard layout in KDE 4?
kde;keyboard layout
You need to tell the system that you have that file. I will not tell you how to modify the files -- you will figure it out -- but I will tell you which files to modify. First, the layouts itself are in /usr/share/X11/xkb/symbols/. You can either copy the file there with a new extension, or modify one of the existing files to add your layout variant. For example, if your layout is based on the German layout, add a section to /usr/share/X11/xkb/symbols/de.Finally, you need to modify  /usr/share/X11/xkb/rules/evdev.lst and /usr/share/X11/xkb/rules/evdev.xml and add information about your layout / layout variant.