If you want to fiddle with the internal wiring of the machine (since you mentioned opto couplers).
Then use the optos you have ordered. The (infrared)LED inside only has a 1.2volt drop.
I would put the opto LED in series with the existing LED in the machine.
No other parts required, but make sure the polarity is right.
If you enable "pullup" in the code for the opto out, then you won't need any other parts there either.
I hope you know what you're doing, because mains power can bite and kill..
Leo..