Pot leaves are stock full of nutrients, especially if you have a well fed healthy plant. Think of all the extra trace minerals and nutrients you feed the plant, ends up in the leaves. If your like me, your canna plants are more well fed than anything else in the garden. So the leaves are some of the richest green material you have available.
I chop and drop my cuttings outside. Mulch, nutrients, worm food, etc.... I think its beneficial, but I also do agree it can raise the risk for problems. As in, if you had something bad already present in your soil, it may speed up the process. As well, the fungus that you don't want to attack your alive plants, may thrive more in canna leaves and stems than compared to say alfalfa(this is just my theory).
I think if your nervous about it, a safer practice would be fully composting or EWC'ing the leaves away from the plants, then apply them to the soil. Myself, I am to lazy, and have to many clippings to collect, so they get chopped and dropped. Large stems, I will cut them up a bit. End of the year, I use a chipper. Chip everything we don't harvest directly on top of the soil. Worm food for the winter!
Back to the OP question.. in a compost tea. Personally I like my compost tea simple. Compost/EWC, Molasses, maybe some fish hydro slate, alfalfa and rock dust if I have it on hand. A purist, microbe tea.
My theory on the bubble hash water. Some of the nutrients are leeching into the water. So the water is like a light version of a nutrient leachate. Similar to soaking alfalfa over night and watering with it.
I think a FPJ, would work great. The leaves get fully digested, and fed back to the soil with increased speed. To lazy this season. I will give it a whirl next year.