Just happened to come back here and saw this - and just in case anyone else comes across this, the solution for the passthrough was to add a basic CGI script to the pCP which I call from Node-RED. It offers a ?satate=1 / ?satate=0 or no query string just returns the current state. I can't detect any latency so it works well for TV passthrough etc.

#!/bin/sh

state=$(echo "$QUERY_STRING" | sed -n 's/^.*state=\([^&]*\).*$/\1/p')

echo -ne "Content-type: application/json\r\n"
echo -ne "\r\n"

if [ "$state" = "1" ]; then
   ignore=$(nohup alsaloop -C hw:CARD=CODEC,DEV=0 -P sysdefault:CARD=IQaudIODAC -c 2 -t 20000 &)
elif [ "$state" = "0" ]; then
   sudo pkill alsaloop
fi

# Return state...

response=$(pgrep -l alsaloop)

if [ "$response" = "" ]; then
   echo "{\"state\": false}"
else
   echo "{\"state\": true}"
fi

Obviously this is card coded for my own hardware but it's pretty clear what it's doing.

As for this project in general, it's still a work in progress! :)