It sounds great, although it will probably take one hell of a lot of cherries, and you have to push them all through that little tiny bunghole with a stick or something. Hopefully you have one or two trees and can dedicate the entire crop for a year or two, or you can order 30-40lbs from hops/cherries direct. I might age the beer in oak with the brett and then rack 5 gallons onto cherries in a year or three. I have found organic tart cherry extract/syrup (from Michigan I think) that came in quart bottles and claimed to be the equivalent of "3 gallons of cherry juice"... I think they cost around $15 or more a bottle. I added one of these to a 5 gallon batch of lambic a few years back, along with a spice called mahlab, which is dried sour cherry pits (available from penzies spices and in ethnic markets with a middle eastern bent.... Turkish, Iraqi, Iranian, Pakistani, maybe Greek but they probably call it something else). The overall effect was good (I would recommend the mahlab... it really adds a nutty thing that reminds me of good belgian cherry beers), but I felt like I would want to double the cherry in the next batch... for your 15 gallon barrel, that would be a $90 investment. I'm not sure how many cherries you can fit in a 15 gallon barrel, but I would try to fill it for the maximum effect and then put a finished beer on top of it for an extended ferment (near a floor drain).