In a structure, is it legal to use one array field to access another one?

Would it be legal to write s.a[6] and expect it to be equal to s.b[2]?

No. Because accessing an array out of bound invoked undefined behaviour in C and C++.

C11 J.2 Undefined behavior

• Addition or subtraction of a pointer into, or just beyond, an array object and an integer type produces a result that points just beyond
  the array object and is used as the operand of a unary * operator that
  is evaluated (6.5.6).

• An array subscript is out of range, even if an object is apparently accessible with the given subscript (as in the lvalue expression
  a[1][7] given the declaration int a[4][5]) (6.5.6).

C++ standard draft section 5.7 Additive operators paragraph 5 says:

When an expression that has integral type is added to or subtracted
from a pointer, the result has the type of the pointer operand. If the
pointer operand points to an element of an array object, and the array
is large enough, the result points to an element offset from the
original element such that the difference of the subscripts of the
resulting and original array elements equals the integral expression.
[…] If both the pointer operand and the result point to elements
of the same array object, or one past the last element of the array
object, the evaluation shall not produce an overflow; otherwise, the
behavior is undefined.