I am starting to sort this through.
Han and Zelandakh, and others, stress that first it has to be decided if 4
♥ could be natural. Han (and Frances) appear to see this as less cut and dried. Justin speaks of "the large time commitment to get it down properly" which I take to be a thought along the same lines.
Here are two auctions:
I (the one I had):
1♦ Pass 1♥ 1♠
2♣ Pass 2♠ Pass
3♦ Pass 4♦ Pass
and
II the uncontested variant
1♦ Pass 1♥ Pass
2♣ Pass 2♠ Pass
3♦ Pass 4♦ Pass
I think there is an important difference.
In I, opener had a clear opportunity to show three card heart support over the 1
♠ intervention. Having not done so, he could bid 3
♥ over the 2
♠ bid to show his doubleton. Whether he would always do this when he holds two hearts and six diamonds is not so clear to me. With, say, Kx I imagine he would. With xx and strong diamonds, maybe he would go with 3
♦.
In II, over 2
♠, it seems less clear to me that opener would be comfortable bidding 3
♥ on a doubleton. It's true that he might feel that if he had 1=3=5=4 shape he would have raised hearts immediately, so with 1=2=6=4 shape he is free to bid 3
♥ now expecting it to be understood. But I don't usually think that deeply during an auction. At any rate, in auction II, it seems to me that at the time of the four diamond bid it is still possible for there to be a playable 5-2 fit in hearts and, in some hands, we might want to play it there. It's not that hard to construct such hands.
Anyway, I have come to agree that the problem is not kickback itself but rather the fact that reasonable minds might disagree on whether a 4
♥ bid could be an offer to play in 4
♥. I now think that thanks to the non-support double and the later 3
♦ rather than 3
♥ bid, 4
♥ is not likely an offer to play. In auction II this is less clear to me.
Watching Vugraph it is not totally rare to see even the top pairs experience confusion over whether trump has been set and if so, what it has been set as. More complex auctions than I am presenting, yes, but then I don't play at their level. As Han rightly notes, I don't have such extensive discussions with partners.
Fwiw, I give you a hand from the other day:
1
♠ 2
♦
3
♦
Uh huh. As it turns out, N has great cards and, further, everything is lying right and you can make 7
♦ (or 7NT for that matter) if you choose the working line (major kings both on the left, spade length on the left). But looking only at the S cards, it is not so clear to me after the diamond raise that the only issue is key cards. So even if 4
♥ were kickback (it would not have been) it is not so clear to me that it is the right call.
All in all, I am thinking more positively of kickback, but Justin's "the large time commitment to get it down properly" still gives me pause.