the wires jumping from one letter to the next can be hidden pretty well if you place the connection points at key parts of the letter where the jump from one to the other is minimal.
you could also disguise the wire with a vinyl close to the colour of brickwork.
as peter has says, you could wraggle the brick cement and have it re-pointed to hide the cable.
if you do have access to the rear just get a decent large masonary drill with 18" long drill bit and drill through dropping the cable through to be daisy chained at the rear. one hole per letter is all thats needed.
if you dont have access to the rear you are really limited with what you can do and regardless to how or what can be done to hide wires, it really does come down to "is the customer willing to pay for this extra labour?"
one thing to keep in mind is the brand/brightness of the led’s used, the colour etc… i would certainly outline that halo lit letters onto a brick wall will be no where close to same on a white board. the drak matt rough finish will absorb a great deal of the light.
i remember years ago doing halo lit onto a brown composit panel and all you got was the reflection of the led’s on the rear, zero glow! and i was using a defuser. nice sign but looked crap at night and i had to hold my hands upto not realising this would happen.