I don’t recall how Kieschnick was handled on the sites I played in in the past, but I think your proposal is exactly how this situation should work. Pitchers don’t currently get credit for their hitting, and a position player who comes in late to pitch isn’t given credit for that either, so I completely agree with the thought that players with dual hitting/pitching eligibility should only have hitting or pitching points accrue for a given day in the lineup.