As East, I picked up:
What's your rebid holding my cards?
I decided to pattern out, and bid 4C. Partner, unsure what this was (cue-bid self-agreeing hearts?) kept the bidding open with 4H and that's how we ended up in 6H for this result:
I should have kept it simple. Knowing that we were going to slam anyway, I should have simply bid 5NT over 3H. Partner could bid 6H with 3 of them, bid 6NT with his actual holding. If partner decides to scramble to 6C or 6D, I can pass 6C or correct 6D to 6NT.
4C, undiscussed, was simply too risky.
p.s. Thanks to John Goold for putting his Bridge Blogging Tools on the cloud. The hand diagrams in this post were formatted using his tools.