This tool collects files into initramfs cpio. And (by default) compresses it. All this happens inside a pipeline. No temporary files. Cool? Maybe.
One of the features implemented already is the ability to drop mkircpio into /etc/kernel/postinst.d/ and have it autogenerate your custom initramfscpio image. In this mode mkircpio needs a one singe configuration file - a list of files, modules, firmware files and directories. See the source. At the very beginning there is quite comprehensive guide on how to use mkircpio.
(Yes. I’ll write better README when the time comes…)