Tapping would introduce shavings into the egr port but they wouldn't go far and should be easily sucked out with a vacuum, no need for oil when tapping a simple hole like this in aluminum so it won't be a sticky oily mess, just a few shavings. Worst case scenario they might get sucked into the turbine blades but they are much stronger than compressor blades and I wouldn't foresee much if any damage to the turbine from some tiny aluminum particles.