Correct, tmp2 of CONV is not used for anything. You rarely need to "blacklist" (that's what you meant) elements in CONV as generally you'll only want to convert elements of a small number of types to some other type. If that number is fewer than or equal to five, you can just layer five CONVs with tmp (the "whitelist") set and call it a day.
We don't generally implement new features for the "why not" and the "beginner-friendly" aspects though. I'm certainly not implementing this for the well-established reasons above, and to some extent because this suggestion falls under the Nothing New point in this thread.
Yeah, i agree with @LBPhacker.
You need to convince devs for why it should be added and not "why not" it should be added.
There are already many ways to do it, just use STOR to isolate DUST from other powders and then use CONV.