Paddy – in addition to Scotty’s comments, whilst an Executive Committee (E/C) can grant consent for the casual use of the Common Property for vehicular parking under By-Law 2, as I get the impression that parking “from time-to-time” is more like most, if not all of the time, then the E/C is really over-stepping the mark.
If that’s the fact, then I don’t believe that it’s possible for affected Owners to seek to obtain a Ruling, as that’s only considered in circumstances where an Owners Corporation has been approached to grant Owner/s a Licence to use its Common Property, and that’s been put as a “special resolution” to a General Meeting and been voted down – and from my reading of your post that’s not what’s happened.
So rather than trying the “exclusive use” approach, that fortunately failed (see last para), I’d suggest a Motion to grant affected Owners a period licence (annual?) under the provisions of Sect 65B of the NSW Strata Schemes Management Act, where you’ll see that “terms and conditions” can be incorporated such as to require benefiting Owners to make payment/s to the O/C.
If such as Motion can be properly passed at a General Meeting, then that should both satisfy the Owner who believes that another carspace is worth something (and they’re right in my opinion also), enable a more formal “trial” of the arrangements, and avoid all the pitfalls of “exclusive use” – including the perpetuity of such arrangements irrespective of changed circumstances, including different Owners or Tenants who may be less accommodating.