Landrover stopped recommending ATF in favour of MTF94 a long time ago. But having looked at the spec of synthetic Dexron VI, it might be OK.
From a thread on R380 lube:
Ideally R380 lubricant should have the original 5W20 low viscosity of ATF fluids to give minimum gear drag. However, it's advantageous for it to be formulated only from fully synthetic base stocks since these will have a much higher lubricant film strength so as not to be squeezed out of those all important high pressure contact areas where wear might occur. This, and the low viscosity, has the added benefit of reducing friction which lowers transmission noise and improves economy.
Google didn't through much light on the subject, though I did find a South African thread in which someone was considering using it.
Only one way to find out - keep us posted...