This would actually be pretty cool. Or that the torches start giving less light so you know a ghast is coming for you.
Maybe a creepy wind sound to go with it...
Good idea!
Or maybe the screen could go all distorted and screaming ambiance would start to be heard.
And you don't actually see the ghast, its a blur that comes screaming down the corridor at you, knocking out all torches as it goes. The only way to stop it would be with obsidian, if it touches it it disappears, thus giving obsidian farming a reason.