Ahh! I'm like three years late to discover a new entry in Apple's review guidelines! (Why this wasn't shouted from the rooftops at the time by the Apple news sources I follow, I don't know!)
My Groundhog Day loop of answering the same email question every day about "why can't I use my unlock key on the iPad?" will finally be over!Apple wrote: 3.1.3(b) Multiplatform Services: Apps that operate across multiple platforms may allow users to access content, subscriptions, or features they have acquired in your app on other platforms or your web site, including consumable items in multi-platform games, provided those items are also available as in-app purchases within the app.
The "Where will my purchase work?" matrix on the unlock page will finally be able to read "Buy here: use anywhere!" So good! (I also just learned a couple weeks ago that apps can finally opt-into having Apple's Family Sharing feature work for in-app purchases, which I did for Synthesia immediately. So many roadblocks removed for everyone!)
The unlock key text box will be in the very next Synthesia for iPad update (which may go out as a small bug-fix style release just to get it in everyone's hands faster)!