When talking about topping up barrels, Vinnie (Russian River) mentioned that the best way would be through a tube to have minimum affect at breaking up the film layer on the surface created by the brett. This does only work for liquid additions though, for actual fruit, just be very careful.
During the recent Heretic Tafflebully show, they talked about adding sugar periodically to speed up the brett. They weren't sure that it worked but it really can't hurt.