I think teleport traps are designed to teleport you very suddenly, without any warnings. That's how it works in my mind and that's what I suppose TB wanted them to do. The existing messages are rather explicit about this: "Suddenly you stand somewhere else" without control, and "The *foo* suddenly disappears!" for enemies, IIRC. You don't see any green light or anything, and I think that makes sense. If it's a trap, it should catch you by surprise.
I think a message would be in order to avoid the confusion of having to move the cursor out of the blue, but the coherent thing would be something simply like:
Control: "You suddenly teleport. Move the cursor..."
No control: "Suddenly you stand somewhere else." (as now).