Isn't the whole "acknowledging the local deities and leaving it at that" basically the default "Pagan and local folklore" religion?
I mean what/how are you converting them? If you preach "Just worship whatever you want" then why/how on earth are they "converting"? How do you convert someone without changing their beliefs?
Remember that no one in BM is truly atheist (although some may care little about religion), so in reality your "worship your private gods and don't speak about it to anyone else" idea is essentially practiced by default by a majority of nobles and peasants in the game.
I think that it is possible to have a "tolerance" religion in BM and for it to be SMA however the point of player-made religion is that it has something different to offer nobles than the ordinary paganism.
Be creative: perhaps the main deity of your religion is the god of order, rulers and authority. Every ruler secular, religious and divine (even other deities) is in power only by his will. You could preach that it is the duty of your followers to respect, acknowledge and follow their regional/family/personal rulers, religions and gods because they were put in place by your ruling deity; however you require converts to acknowledge that the authority of these other deities comes directly from your god, and to worship him as well.
Of course this is just an idea; I'm just demonstrating that there are a myriad of interesting ways you can do this. However I think that just to create a religion of not caring about religion is pointless and boring; doing nothing to enrich the game, especially when a religion is something that can add so much interesting rp.