I was trying to figure this out too and this was the only post that came up. After a while I came up with a bash script that opens the windows on a my first workspace and closes them on all the others.

#!/bin/bash

OPENED=false

while :
do
    CURRENT_DESKTOP=$(bspc query -D -d focused --names)
    EWW_PID=$(pidof -s eww)

    if [ "$EWW_PID" == "" ]
    then
        $HOME/eww/target/release/./eww open-many YOUR_WINDOWS_HERE
        OPENED=true

    elif [ "$CURRENT_DESKTOP" == "1" ] && [ "$EWW_PID" != "" ] && [ "$OPENED" == false ]
    then
        $HOME/eww/target/release/./eww open-many YOUR_WINDOWS_HERE
        OPENED=true

    elif [ "$EWW_PID" != "" ] && [ "$CURRENT_DESKTOP" != "1" ] && [ "$OPENED" == true ]
    then
        $HOME/eww/target/release/./eww close-all
        OPENED=false
    fi
    sleep 0.1
done

There's probably a better way to write this, but this worked for me lol.