He Guys,
I am user of corona for Max for a long period of time, and using most of the other render engines, but this feature is just must have, I do not know why your team decided to not include this here, I guess Corona in C4D is just only render who suggest workaround with emissive material for this. First of all, light is much easier to manipulate than geo planes with material. Just include this option in light as well, two slots, one for color, another for alpha. Or just checkbox to use color texture as alpha.
Best,
Ed