So an update on this just for the next person who comes across this thread, since everyone is always looking for a post for specific machines. If you've got a Fanuc based live tool machine with subspindle, the HAAS ST series posts are really close and well made. I had to change how some drilling cycles were handled, of course had to change out all of the Mcodes and such, and had to add some additional formatting rules (all caps on things, different decimal behavior on peck drilling, etc) but altogether it took me about a week of on-and-off work to get my KIA SKT21LMS up and running with all features from the HAAS ST20 post. It was a good bit of trial and error, and some careful testing, but I'm really happy with the result and the subspindle and C axis milling works way better than I could have ever made it work by modifying the stock FANUC post.
Now that I've been in there a few times, I am pretty confident I can add just about any functionality I want to any of my machines.
So don't be afraid to get your hands dirty! It's really empowering when you are in full control of your software and machine, and can program and run with 100% confidence since there is no hand coding required.