I used black locust for grape vinyard posts. Locust grows tall fast, but doesn't thicken very quickly. However it is a very sturdy tree and I think will coppice well. I think you'll get 4 or 5 year coppicing cycles from them.
One thing to consider...locust MUST be thoroughly cured before using it as a post, it grows quite readily from cuttings and fence posts have a reputation of suddenly sprouting and becoming a row of trees. I can't at the moment remember what this cutting method is, but I used it extensively to grow a mulberry hedge row. 3 ft long arm thick logs planted 2 ft deep into the soil in the late fall.
keep in mind, coppicing as been traditionally used to grow firewood more than anything else, which doesn't have to be post thick. You can get 1-2 inch branches off a coppice every year or two which is much faster than waiting for 3-5 inch thick post quality wood.
EDIT: the type of cutting is called a truncheon...golly I couldn't find it on the internet then I realized I used to write a bit about it on the internet and had to look up my own name mulberry and cutting and there was something I had written. Golly I hate getting old. Lol...no you aren't going to get my name :)
Something else to consider. Locust is very prone to throwing up suckers, and not necessarily near your trunk either. A new tree can sprout from the roots several yards away from your tree and it's growing from the root which means you can't get rid of the sucker without cutting the root off AND pulling that runner root out of the ground cause it won't die and will just throw up another sucker somewhere if you don't. As majestic as locust is, it can also be a right pain in the arse.