Hi,
from my understanding, corona utilizes the Diffuse channel to be displayed in the viewport. what i do is cpoying the opacity texture to the diffuse channel for placement. after that i simply set the mix strenght to 0% or delete the texture in diffuse. Not sure if you have to deactivate the opacity channel while placing your decal though.
Not the smoothest way of doing it but i see no other solution right now.
Maybe this helps ;)
Greetings,
Moritz