I think 4
♦ is the bid. I too am initially attracted by 3NT, as it will very likely make. But we are covering most of partner's losers in the minors as well as a control in hearts and trump finesses through the preemptor's partner - it looks like we will make quite a few tricks in spades.
Even with no LTTC agreement, I think opener should still bid 4
♥ with some hands that don't contain a heart control but are still interested in slam - makes the whole auction a lot easier and might divert a heart lead, which adds some positive weight to factor in against the negative effects of deceiving partner. One example of a "fake last train" hand might be something like
NB: I think most of the hands that fit this bidding pattern will have (very) strong trumps, as is it quite hard to construct a hand where we want to suggest slam without a heart control or good trumps. Also, good trumps mean that we can be optimistic about slam, as we know partner has made a try probably looking at 3 or 4 small.