Yeah NAM, you can set it to events or player. So as long as they are actual events and not just sprites from a script. And yes, terrain tag is taken from the highest layer with a tag.
That's not for you taichi. Just for NAM if he's interested in the method to get terrain tag.
Anyway, so yeah, if your shadow is it's own tile, you can give it a terrain tag and it will take it. You'll only really encounter problems if (a) you need more than 7 (8?) tags or (b) if you want two identical tiles (as in identical layers too) to have the same tag. Though actually, you could use a transparent tile with unique tag on the 3rd layer if you didn't need the 3rd layer on that tile.
(a) could be solved with a minor script
(b) could be too, but configuration might be a bitch. Both would be annoying actually, but (a) would be by tileset and (b) would be by map