Spot on! :) With the help of opDispatch (the catch-all member function temlate), it's possible to drop the string from the use site. (I don't know a way of dropping it from the type name.)

  import std.stdio;
  import std.typecons;
  import std.string;
  
  // This type's opDispatch removes the need for string in the flag name.
  struct FlagFromBool {
    auto opDispatch(string flagName)(bool value) {
      mixin (format!q{
        return value ? Yes.%s : No.%s;
      }(flagName, flagName));
    }
  }
  
  // A convenience function to remove the need for empty struct construction parenthesis.
  auto flagFromBool() {
    return FlagFromBool();
  }
  
  // Unfortunately, the type name still requires string flag names:
  void bar(Flag!"foo" flag) {
    writeln("called with ", flag);
  }
  
  void main() {
    // However, the expressions don't need a string:
    bar(flagFromBool.foo(false));
    bar(flagFromBool.foo(true));
  }