Brick Mailbox with planter on top
If you decide to make a planter, I would line it with a water impermeable membrane, liquid applied is probably the best option. You will see brick deterioration over time without it with water sitting on the backside of the brick. Not to mention the efflorescence you would get on the face.
The plan for drainage sounds good, but makes sure you have a way to prevent debris, dirt, or anything else from clogging the line.
On a design note, I would not make a square with a planter on top. I think that would look weird. Instead, I would plan a tiered shape, with a planter about mid way up the back side and have the mailbox tower get thinner from that point to the top.