Yes, stacking materials in corona is frustrating cause in it's current stage is not supported. I have seen on their road map that it's to be implemented next release. wohooo :)
Regarding your problem with material stacking. I found a dirty solution, I had tested quite a bit on simple mesh and also in hard surfaces and its been working flawless.
Basically it has the same concept as the material stack but instead I stack geometry.
1-Add have the base material on the your object
2- Duplicate that object and delete the base material in object manager(you can just polygon select the part of geometry you will add the stencil and press u~p to split)
3- Create the new material you want and assign to the duplicated mesh. Do not forget to check and feed the opacity channel with a black and white image or shader.
4- Adjust the uv projection, placement and scale to your needs.
5- I tend to use the image or shader I used on the opacity channel also in the displacement channel with a max value 0.1, 0.05 or even less to pull it out in case its overlapping with the base material.
Hope it helps