I don't know the legal standpoint for this, but would agree that, yes, ethically at least the new school should certainly know what you're appealing for! And I'd be very surprised if this wasn't the case legally too.
Do you have a solicitor/advocate? If so, what's their view?
Also wonder if you could ask SENDIST too, for the legal obligation from the LEA about this?
Slightly aside from this though, have you talked to the school about having ABA there? if you are sure this is the right school for dd, you need to be sure that they'll support ABA willingly, as even if you win it in appeal, if the school is not onside and supportive with the approach, it could be difficult long term. Do they know about what you want, and are they happy with it?
Also, have you submitted your appeal papers? ie does the LEA know exactly what you're asking for? If so, I can't see harm in the new school knowing too. But if papers not submitted yet probably worth keeping your cards close to your chest just for the time being and not giving anything away just yet that might help LEA prepare their case against you.
We were in same position as you, appealing before ds started school. But we had chosen school on basis that they were supportive of ABA (at that point, before LEA got to them!). There's no point choosing a school who will close ranks about ABA.
Also worth considering possibility that the LEA have in fact talked to the school about it, but just not officially...