Really depends on the L brackets. The masonry will hold a static load easily, with a decent anchor. The L brackets will be the weak point. Most that I've seen are nearly symmetrical so they can use thinner material with a strong cross section. It doesn't look like you have that much room under there for the back plate of an L.

I think you'll have trouble finding a decent off the shelf bracket, but it's not a huge deal to fab something.