My understanding of the manual seems to say that only scripts go into the "files" directory and the are only executed after the OS image file has been successfully flashed to the router. The scripts are copied to files/etc/uci-defaults and excuted in the numerical order of the script file names. The 99 has the 99-asu-defaults executing last. If the script was successfully run and exits with a zero then it gets deleted by the system and a copy remains in /rom//etc/uci-defaults/.
The whole thing is real simple and smooth the the WEB GUI but I haven't found anybody posting an example of their successful Linux CLI command.