Thanks for the answer! Perhaps someone could write a script calculating what percentage of common words can be expressed unambiguously using this keyboard? It seems like the percentage would be quite low to me (considering e.g. 80% to be "low" given how often you'd have to issue slow corrections), but I might be misunderstanding the rules and I expect it would improve in English with your Z-as-S idea.
The general idea has been used for decades in Europe by people doing closed captioning for live television and other types of speed-demanding work. So the idea is tried and true, and I have no worries about ambiguity from that perspective.
The bits I'm uncertain about are the specific details around substitutions and such.
Maybe it makes sense at least in some instances to repurpose the Z as a plural S, but I suspect it will be somewhat dependent on what comes before it.