Hi,
Indeed. It's bit of a shame really, I agree on this. I think it's possible, only that's it's not efficient and therefore not really something to spread out in the wild withotu caution. Relying on the z depth will definitly perform as expected, so I think it's best to stick with it, and maybe contact Unity to push them. They really take feedback seriously, so the more they know about what's expected, the better.
Bye
Jean