this way seems a lot like poor mafia sportsmanship. One should not be discussing anything related to the game via PM. By extension, one shouldn't discuss PMs in the game, as your mutual referencing of a message that contains non-public information could be seen as talking in code.
I think it's fine (well I thought about it before I did it, and decided it was fine, otherwise I would not have done it). Firstly, it's obvious that the other way is BM. You can't discuss the game, or anything related to the game outside of the game, because that's just the rules. However, I don't see a problem with discussing outside stuff inside of the game. We discuss avatars during the game, and noone has a problem with that. Chairs mentioned stuff about his kid, and noone has a problem with that, and I'm sure there are other examples.
You made a point about non-public information. While I totally agree that you could figure out a way to cheat like this, I don't think that's reason. There is actually zero assurance that players don't cheat. If you want to cheat in f.ds mafia, you don't need to invent codes or send hidden messages, you can just do it via pm, and there is nothing that can stop you. You just have to trust the players not to do it.
And another thing is, even if you ask something about mafia in general, it's not related to the game. If you look close enough, you'll see that you do this all the time. You saying that it might not be okay is not related to the game. You could have done that via pm.
So, I really don't think it's a problem.