You are talking about lambic yeasts, open fermentation has been a "thing" since at least the 11th century and probably earlier. Lambic or wild yeast are used in very specific styles, I don't really see the point of adding a second yeast to the bottle, adding sugar to the bottle is just priming. This part is all opinion> Also I wouldn't mash in the barrel, I don't really see a way to keep it up to temp, and if you over temp, the tannins from the oak might ruin your mash.
Thats all I have for you, Protester should be able to speak on this from a more educated place than I.