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

Because of the copy that append might do, you now have 2 copies of the key in data, but you only scramble one. There are many functions that might make a copy of the data given that you don't manually manage memory in Go. And if you are writing an open source library that might have dozens of authors, it's better for the language to provide a guarantee, rather than hope that a developer that probably isn't born yet will remember not to call an "insecure" function.