I had also "heard" they would do it in the LHS.
So they join as an F/O, they train in the LHS, thus can only fly on the line with C+T, so that will not go on any longer than it needs (if that long at all), then they get signed off to fly with F/O's when they are still sitting in the LHS (sounds oddly similar to a 3 bar check)...
hmmmm.....
Let me think about it, they never fly in the RHS, the first time they fly on the line once out of the training system is as a Captain, but they are not DEC's?
If it walks like a duck...