I like it. Assuming the bmx bar isn't compromised by all that rust (throw some oil inside, as it'll collect rain) it should be pretty sturdy. Copper is fine for the shim, where it's ductility will not be a problem.

Actually, what i think would be even better would be to cut the bmx bar a little higher, and have a second set of normal bullhorns coming off above the faux road drops- it'd give you a narrow and a wide hand position.