When I did more western showmanship I had a gaited/morgan cross with a gorgeous long mane. I tried a lot of different things over the years that I showed him, but I usually either left it loose (shampooed, tangle-free, and trained to the correct side, but loose), or banded it long (got some odd looks, but it worked). A running braid should work, but is more conventional in English G&S divisions on Morgans, H/J Gaiteds, or Baroque breeds.
One thing that I never got to try was starting the diamond continental pattern, getting one row of diamonds down the crest, and letting the rest of the mane flow. I'm waiting on Scout's puffy pony mane to get long enough to try that one.