HOWTO customize XKB without modifying system files

I have finally succeeded in my quest, to get XKB to easily set up the keyboard in an arbitrary fashion, not limited to the options presented in the rules database, using the system database for everything I'm not explicitly overriding, without needing to edit any system files or have any special privileges!

This should allow me to map CapsLock to Super (or Esc, or something else, I'm still playing) on any computer I have an account on.

( Most of you probably don't care about the details... )