Wouldn't that be just as hard as for any other function? The name of a function (a bit like the name of an array) evaluates to basically a function pointer value, there is little difference between the two calls here:

    int foo(int a, int b)
    {
      return a + b;
    }

    int main(void)
    {
      printf("Direct: %d\n", foo(1, 2));
      int (*ptr)(int, int) = foo;
      printf("Indirect: %d\n", ptr(1, 2));
      return 0;
    }

of course default arguments, if added, would have to be part of the function pointer type as well, making the above:

    int foo(int a = 1, int b = 2)   // NOT REAL CODE, FANTASY SYNTAX
    {
      return a + b;
    }

    int main(void)
    {
      printf("Direct default: %d\n", foo());
      int (*ptr)(int a = 1, int b = 2) = foo;   // NOT REAL CODE, FANTASY SYNTAX
      printf("Indirect default: %d\n", ptr());
      return 0;
    }

Unnamed function arguments would look silly (`int (ptr)(int = 1, int = 2)`?), but I would be radical then and only support default argument values for named arguments, probably.

Edit: fixed a typo in the code, changed in-code comment.