os0:psp2config.skprx once decrypted is a UTF-8 text file that is parsed by SceSysStateMgr. It is a very simple script format.
If "Producting Mode" (MANUFACTURING_MODE) is enabled, then it is possible to load psp2config.skprx from sd0: (SD adapter in GameCard slot) or ux0:.
However, the files must still be signed and encrypted SELFs.
In "Producting Mode" if sd0:psp2diag.self or ux0:psp2diag.self exists (and is a valid signed & encrypted SELF), then it will be launched.
If "Development Mode" (DEVELOPMENT_MODE) is enabled (note that this does not necessarily mean PDEL or devkit), then psp2config.skprx can be a plaintext file instead.
(perso, j'ai pas tout compris)