I guess that to annouunce it in a polite way is ok. At least I would be ok with this. To ask to play it out, I would prefer to ask it when the game is unrated. Because nobody wants to be mated. :D
I think the general rule would be "do what you'd like others would send to you" or something like that, IDK. :)
Is it rude to announce when you have a forced mate? Recently I was playing and had a forced mate in 8, which is by farthest ahead I've ever calculated a forced mate, and wasn't sure if it was rude to announce it. Also, is it rude to ask to play it out? It was a smothered mate and would be the first time I would've pulled one of those off in a game.