As I understand it, this is too brittle. I think this is trivially defeated if someone adds an append to your code:

    func do_another_important_thing(key []byte) []byte {
       newKey := append(key, 0x0, 0x1) // this might make a copy!
       return newKey
    } 

    key := make([]byte, 32) 
    defer scramble(&key) 
    do_another_important_thing(key)
    // do all the secret stuff