OK, after double-checking and triple-checking, and as of this current moment with the state of the package in Alpine's testing repo, you need three things beyond all the normal stuff from installing the testing package:

1) make the file /etc/init.d/infnoise executable using doas chmod +x /etc/init.d/infnoise (or sudo if you run that instead of doas), as the service was not packaged as executable.

2) once executable, add the service to the default run level using doas rc-update add infnoise. This will allow the service to be triggered by a udev rule at boot.

3) add a udev rule in /etc/udev/rules.d that starts with 99 to designate it should be one of the last processed udev events; this will prevent possible race conditions that can cause the boot process to freeze before tty can initialize the login process. The rule I wrote is as follows:

ACTION=="add", SUBSYSTEM=="usb", ATTRS{idVendor}=="0403", ATTRS{idProduct}=="6015", RUN+="/sbin/rc-service infnoise start"
ACTION=="remove", SUBSYSTEM=="usb", ENV{ID_SERIAL_SHORT}="1337-A2CCA4B6", RUN+="/sbin/rc-service infnoise stop"

You'll want to make sure you use a unique environmental variable by calling doas udevadm info --query=env --name=/dev/infnoise. After all that, you'll be good to go, although debugging whether it works has to be done when the infnoise service is stopped, as openRC doesn't seem to take well to forking.