No, there is no way to specify defaults. I believer this is done on purpose to enhance readability, at the cost of a little more time (and, hopefully, thought) on the writer's end.
I think the proper approach to having a "default" is to have a new function which supplies that default to the more generic function. Having this, your code becomes clearer on your intent. For example:
func SaySomething(say string) {
// All the complicated bits involved in saying something
}
func SayHello() {
SaySomething("Hello")
}
With very little effort, I made a function that does a common thing and reused the generic function. You can see this in many libraries, fmt.Println for example just adds a newline to what fmt.Print would otherwise do. When reading someone's code, however, it is clear what they intend to do by the function they call. With default values, I won't know what is supposed to be happening without also going to the function to reference what the default value actually is.