It works poorly at any speed. Hi-Z is an undriven signal, not a specific level, so voltage-driven logic like (C)MOS can't distinguish it from an input that's whatever that signal happens to be floating at. In current-driven logic like TTL or ECL, it's completely equivalent to a lack of current.

I wasn't pitching it as a solid commercial idea. Just that you can get (perhaps fiddly) three states out into the real world with something cheap that already exists. Like: https://idle-spark.blogspot.com/2015/04/low-cost-n-ary-dacs-...