There are a couple of other well-known defensive metrics that attempt to implement a zone on seasons without a Zone Rating. One is TotalZone, developed by Sean Smith (AROM at Baseball Think Factory), and the other is Dan Agonistes' Simple Fielding Runs.
Both of these estimate a Zone Rating using the Retrosheet play-by-play data, which only goes back as far as 1953. That's fine, but with the current rate at which new seasons are added means we have to wait another thirty years or so to get all Major League history covered. I don't think I can wait that long, as I'll be near 80 by the time we get there. And I don't expect to live to such a ripe age.
In the meantime, we need another system, which is where I think my Defensive Winning Percentage wins out. With a few tweaks, I think I can improve it for the pre-1954 period, when it is hampered by not knowing the number of doubles and triples a pitching staff has given up.