Can GCC warn me about modifying the fields of a const struct in C99?

A way to design your way around this, if needed, is to use two different types for the same object: one read/write type and one read-only type.

typedef struct
{
  char *ptrChar;
} A_rw;

typedef struct
{
  const char* ptrChar;
} A_ro;


typedef union
{
  A_rw rw;
  A_ro ro;  
} A;

If a function needs to modify the object, it takes the read-write type as parameter, otherwise it takes the read-only type.

void modify (A_rw* a)
{
  a->ptrChar[0] = 'A';
}

void print (const A_ro* a)
{
  puts(a->ptrChar);
}

To pretty up the caller interface and make it consistent, you can use wrapper functions as the public interface to your ADT:

inline void A_modify (A* a)
{
  modify(&a->rw);
}

inline void A_print (const A* a)
{
  print(&a->ro);
}

With this method, A can now be implemented as opaque type, to hide the implementation for the caller.