I think part of the problem here is a bad question and an equally bad assumption - viz : "I would have thought the engine out speed was minimum drag (to minimise altitude loss after engine failure)"
This is just not the case. EO speeds are a compromise between acceptable single engine cruising levels and obtainable TAS's. The TAS thingy is of course a function of your single engine 60 minute range from which your ETOPS certification is derived. There are quite significant trade offs in each direction (altitude/fuel and range) but most operators seem to hold the 767 around the 410 TAS mark (local mileage may vary).
Pure driftdown performance to optimise altitude is another beastie altogether. It is rarely taught and even less often understood and it is a big topic. However you are right in that the en route holding speed and pure (max altitude - min ROD) driftdown speeds are closely linked - in that they both have the requirement for min power (and thus FF in the holding case).
Min Drag Speed is another value altogether and is below the holding speed. The problem is that with all but a small number of airframes Vimd is a speed unstable target. In that it is on the lower cusp of the prevailing drag curve and any speed deviation tends to make the drag component vary in the wrong (unstable) direction. So Holding Speeds are elevated away from this point - usually around 1.1 Vimd. Since the bottom cusp of the drag curve is fairly flat in this region this small speed increase has little effect on power (FF) required and has significant benefits in speed stability.
So the summary would be. For range and ETOPS considerations, use your published drift down speeds to achieve published TAS's and ETOPs ranges. If you are worried about Single Engine driftdown altitudes, look at the en route holding speed for your present altitude and come down on that. Once your driftdown problem (in the altitude sense) is complete - and they rarely last long - convert to SOP speeds to fulfill your SE requirements of speed and range.
I may have grasped the wrong end of the stick here, but, hopefully I have brought a little illumination.
MG