You’re in luck. As far as i recall wincompose is inspired by Xorg’s Compose feature (https://github.com/samhocevar/wincompose?tab=readme-ov-file#features). Depending on your installation you can toggle it via some settings, or by running: setxkbmap -option compose:ralt
I always find the book useful for this stuff: https://wiki.archlinux.org/title/Xorg/Keyboard_configuration#Configuring_compose_key
What’s the use case? Is there a reason that the disk is not unlocked at boot/login?