The Argon ONE is “only” a enclosure with an integrated MCU which observe the IR receiver and the switch on the back side for a power on/off signal. If you expect something like WakeOnLAN, this would be part of the RPi5. Also if other HATs available for that, I don’t know how many power you could save in that way, compared to an RPi5 always on in idle state.
It’s possible to reprogram the IR power on/off (non-discrete) signal via a script, which is provided by the argon1.sh installation. But I don’t know if the MCU firmware inside of the case support other protocols than NEC as well like your desired RC5.
If you have a universal remote control or something like that, you can use the corresponding lircd.conf and the toml with the known OBC (original button codes) of the Argon IR remote as a starting point.
You can also find a Pronto configuration here: IR pronto codes for Argon One?