I have battled with Bermuda grass many times but, while I have never done battle with it with a hugel bed, I found that if I have a good weed barrier and a "thick" bed (18-24 inches) it doesn't come through. I always do raised beds though with some sort of raised sides. I hope that helps a bit.

Bermuda is going to invade every thing from flower beds to flower pots so you just have to stay on top and remove before it overtake's. My approach is burying a treated 2x8 on edge about 2 feet from mound. That make's it easier to notice first invasion so it can be removed. Don't tell on me but I've been known to use herbicide on 6" path outside barrier.