"Output GND" as in the secondary side GND, not mains neutral.
You're proposing a single-wire indicator, so what on earth do you mean by "the secondary side"? Please draw a diagram to explain what you're proposing, you'll end up with at most a diode between mains earth and mains neutral = not good.
I'm aware of that danger now. I thought c4757b may have misunderstood me.
Before anyone else gets worried: if I had known the answer I would not have asked the question. No I'm not working on any mains-side circuity.
There's nothing wrong in not knowing enough about safety. There is a lot wrong however in pretending that you do if you don't. I'm being open about what I didn't understand: that's why I asked the question here in the beginners forum
. The worst thing anyone can do is give people a sense that they should not ask safety questions, especially if they seem dumb to others.