Have you tried to set the default values on the local block through the shared block's SetDefaultValues?

Example:

public override void SetDefaultValues(ContentType contentType)
{
	LocalBlock.PropertyOnLocalBlock = "value";
}

You probably know this, but also be aware of that SetDefaultValues only kicks in when creating the content type.

Yeah your suggestion does not work. But I did find a solution ... strangely enough this works:

public override void SetDefaultValues(ContentType contentType)
{
    LocalBlock.Property["PropertyOnLocalBlock"].Value = "value";
}