The invisible window exploit from leaf blocks wouldn't do much, it'd only hide leaf blocks. Just like you can see between the leaves now, if you make them completely transparent, the leaves will be invisible. Since there aren't leaves underground you can't really use it to see underground, unless it hides all blocks behind them, in which case you'd only see the void. That's what my logic tells me at least.
But no, it's not legit to do that. I can make the map, I guess I should map the world soon anyway. Idk when it will be finished.