Baking decals would imbalance the benefits you get from them. In the case of overlapping them on a tileable texture, for instance a brick wall with graffiti you would have to bake the whole wall texture + the decal and merge it into once big texture. You would dramatically increase texture memory usage.
The only way this is feasible would be through virtual texture maps. You can layer in texture samples and mix them with a lower gpu footprint. But in this case it probably wouldn’t be a decal but a static mesh projected into the texture.
Something like this:
But instead of a road you would have your fake decal.