Best way to ask for permission for a cameo appearance?

So,

I'm working on a couple of my other stories and in one of them I really REALLY would love to borrow a character or two from another story I have read here, but it has been a long time since it was updated and I'm not really sure that the author is even lurking here any longer (Or even writing for that matter).

Is there a way to use this with proper accolades given to where the character comes from? Or do I just nix the idea and not give the cameo and shout out to those other awesome stories?