If you have the ability and tools to make one, why bother? Seems it would be easier to just modify most attachments to run backward. I was being "set up" to build one, but the owner figured out how easy it was to just modify most of the stuff he used to run backwards, so he dropped the reverser idea. We didn't do a post hole digger, but that is a simple fix, by just going with a hydraulic unit. Changing an existing one over to hydraulics is probably also possible, if you already have you eye on a pto drive one.
He is currently enjoying the rebuilding of the his reverse running tiller, as it let loose something inside and the fun began!
David from jax