I'm using my 12.9 1st gen cover on the gen 2 and it works 99.9% perfectly. It doesn't work with the flash (don't care), it does cover the mics to SOME degree (don't care, as voice dictation still works great if you speak just a slight bit louder - easy to compare - it's very slight), and the camera cut out works just fine. No issues.
Also, nobody maybe has noticed, but there's only 8 speaker holes per speaker on the new iPad Pro 12.9 (so 4x8), whereas the old iPad Pro had 4x9 speaker holes. Thankfully, the 8 holes line up perfectly on the original back case with the 8 of 9 holes there. The 9th hole you just see the metal of the iPad through. Hah. 😀
So... yeah. Bummer there's not a dedicated second-gen back case, but I'll hold onto my original one as it's gold dust to me.