Well I got a way to do it with the OctoRelay plugin, but the indicator doesn't work. It reads the pin state so unless you change the code it shows the real state.
It still does what you want. I also looked for a way to do everything in the plugin, because I don't know if you feel comfortable with ssh.
First the technical side - those are the bash commands we use:
pi@octopi:~ $ echo "17" > /sys/class/gpio/export
pi@octopi:~ $ echo "out" > /sys/class/gpio/gpio17/direction
pi@octopi:~ $ echo "1" > /sys/class/gpio/gpio17/value
I use GPIO 17 for the relay in this example.
First we export the pin we want to use, then we set it to output and the last one turns it off (the logic is inverted).
So what we do now is to enter
echo "1" > /sys/class/gpio/gpio17/value
into the OS Command ON box.
Then we enable another relay (which we're only use to execute the other two lines once) and enter
echo "17" > /sys/class/gpio/export && echo "out" > /sys/class/gpio/gpio17/direction
into that OS Command ON box.
Set the other relay to an unused pin like 4.
Now just click on save and we're ready to go.
You now just have to enable the second relay just once every time you restart the pi and relay one should just send a short pulse.
I also tried the active on start option for the second relay, but for some reason the OS Command isn't executed that way.
If you feel comfortable with ssh we can also look for another way that doesn't need the second relay workaround.