Acai is a source of vitamin C and other antioxidants, so it's good for your over-all health. Will it have a large impact on your health and how MDMA effects you? Doubtful. If you're interested in getting tons of antioxidants, red beans (the Japanese kind) are actually the highest antioxidant potency food (ounce for ounce). Every year, there seems to be a new magical berry that's more exotic than the next, that gets sold as a wonder supplement for weight loss and all sorts of issues.
Antioxidants are beneficial at any stage, so I wouldn't restrict your Acai intake at all- take it before and after.
There is truly no supplement or behavior that can make MDMA more fun than time. Piracetam as a pre-loading tool is amazing for most who try it, and it seems to also eliminate the come-down quite a bit, but nothing can ever substitute taking time off in between.