I played a non-plotted Beastbond Outcaster while controlling a 6/4 Kraul Whipkracker. BbO enters, draws a card, then reinforces itself to a 6/6. Using Tyvar, Jubilant Brawler’s Graveyard Brawl brought another back, making it a 9/9, draws another card, but doesn’t reinforce again to a 12/12.
Further observation: if I play BbO in its own, thus not getting the effect, then play a second one, the effect will work that time. It seems like the reinforce only works once. This is true whether playing them from the graveyard or hand.