Yes, the trouble with passing is that partner may read something into it other than an attempt to find out what is going on. If he meant 5
♣ as a control with hearts agreed, he may read a pass as something - perhaps second round control with hearts agreed - and could even plunge into slam if he has three aces.
I only polled one player, who said that 5
♦ seemed obvious. His view was that a 4
♥ bid only showing four hearts was a little like FILM or Dixon where a bid of hearts is really takeout with the minors as well, so 5
♣ was looking for a minor - and pass would not be a good idea!
The trouble was, when I suggested the possibility of 5
♣ being a control, he agreed that was a sensible alternative, and that made 5
♥ the obvious bid! See, I only needed a one man poll to show there were alternatives!
The big problem is that players are unlikely to have an agreement in this area, so it is reasonable for a sensible responder to make a cheap bid because it is safer and more likely to find out what is going on. So to go back to the OP, I am beginning to think pass or 5
♦ could be ruled as Amber, but not Red.
This is West
W . N . E . S
. . . . 1NT 2♠
3♥ 4♠ 5♣ X
?
1NT is 12-14, 3♥ is forcing with 4+ ♥