Halter training a bull is time well spent.
One of the first steps in training a bull to walk on a halter is what I call "post-training". By tying a young bull to a post, they learn that pulling the rope is pointless. This creates a psychological barrier that deters them from pulling people around as they grow.
Please be cautious -- the psychological barrier is NOT foolproof. A bull is a powerful animal that could injure or kill a person at any time. They should always be treated with respect. Thankfully, Dexter bulls seem to take to this training well. I have personally not experienced any issues with walking bulls I have trained on a halter.
I probably wouldn't lead a bull through a pasture full of cows in heat, but I have on several occasions led an adult Dexter bull through the heifer pasture to the squeeze chute without incident. I do tend to have someone walk along with us with a bucket of his favorite treats just in case. We have yet to utilize the bucket.
You'll have the best success with halter training your bull if you use an appropriate breaking halter. I prefer the Weaver Breaking Halter.
Although inexpensive, I have found that this halter serves me very well with any work I want to do with any Dexter -- including bulls!