Ah I see - what a pain. How many hours ABA do you have booked that day? If enough could you then hire a day nanny from an agency to cover NT?
If you don't have a familiar adult for SN I think you're stuffed really (DH's grandfather is very ill at the moment - he's a (short) plane ride away, but we've discussed it and I think it will be impossible for me to go, so I sympathise). Will dh's family be understanding?