In reply to tehmarks:
> She'd also prefer it to be steel for personal peace-of-mind reasons.
Regarding this bit - if she thinks an aluminium carabiner won't be strong enough, that's an irrational fear and possibly one worth working at to get over.
If size/weight are important, two DMM Phantom screwgates back to back would still be less than half the weight of a steel screwgate/twistlock/whatever.
If there's time to do it up and check it, I'd second (third) the suggestion of a maillon. You'd be wanting a proper PPE rated one, available down to 7mm in stainless steel and ridiculously strong:
http://www.peguet.fr/gb/produits/EPI_industrie-EPI_montagne_normal.html