Actually, after I posted that I realized you do not need a Tiagra rear der but ANY 11 spd Shimano der is OK. I was thrown because (new) Tiagra is 10 spd so I initially assumed they had a special ratio rear der whereas in fact it is the shifter which has a special pull ratio unique to Tiagra and the der is geometrically identical to all the Shimano 11spd models. However, this setup will result in a very slight overshift (ie the der will move slightly too much with each shift) - normally this is not too bad as there is a bit of slack built into the designs but you need to keep adjusting them more often. Shaving the cable pulley a smidge reduces its diameter and thus the amount of cable pulled each shift and makes perfect..
And note if you don't want to butcher your precious campy lever, those models that used a pulley like in the pic above is part number EC-SR060 and they may still be available.