If I were trying to work up a low pressure load in .45 Colt the first powder I'd try would be Trail Boss. I've never used it, but I've read very good things about it in .45 Colt. I've also read that Titegroup is also good for situations where you have a small amount of powder in a relatively large case. Again, I've never used it.
I've used 231 and Unique for sub-1000 fps velocities, and they worked well. My favorite powder in .45 Colt though is 296, but then, my gun is a Ruger.
Mike