I think that your original suggestion - use memcpy is a better solution then volatile.

It looks like the following code would work:

  #include <stdio.h>
  #include <stdint.h>

  void f(volatile uint32_t *x, volatile uint16_t *y) {
    *x = 5;
    printf("%d\n", *y);
  }

  int main() {
    volatile uint32_t x = 10;
    f(&x, (volatile uint16_t*)&x);
  }

And the compiler is guarantied () to issue store op on x = 5 and consecutively load op on y, but the code is looking pretty ugly.

() assuming no alignment problems