Pretty sure it's safe! Take this program:

    typedef struct {
      int x;
    } Base;

    typedef struct {
      Base base;
      int y;
    } Derived;

    int f(Base* b, Derived* d) {
      b->x = 0;
      d->base.x = 1;
      return b->x;
    }

Notice that if we are accessing the base members of "d", we are still accessing them through a struct of type "Base" (d->base.x). If we compile this with strict aliasing, you can see the output is allowing that the two might alias (while this isn't a proof, it's a strong indication that this is aliasing-correct).

    0000000000000000 <f>:
       0:   c7 07 00 00 00 00       mov    DWORD PTR [rdi],0x0
       6:   c7 06 01 00 00 00       mov    DWORD PTR [rsi],0x1
       c:   8b 07                   mov    eax,DWORD PTR [rdi]
       e:   c3                      ret