[With_Two_Flakes]

Here in the UK, our mechanics have been the same for a number of years...

6/7 man crew - LJ stays at LOS till pass is thrown. He takes the Illegal pass call. he has the down box opposite him and he is in a great position to make the call.
4/5 man crew - Ump moves up to LOS on passes. He takes the Illegal pass call. If it is between U and the down box then flag it when you see it. If not, bag it and check it after the play is over, then flag it.

We feel U is in a better position to see this than R who is trailing the play. Yes the U has other duties on passes (ineligibles, inside-out help on complete/incomplete) but clearly the illegal pass would take priority.