Assuming you believe that dosage is valuable in ascertaining how good a horse will be at staying, I don't see a problem with applying it to National Hunt races*. Stamina is an attribute, and although the categories correspond to distances on the flat, the CD and DI give an indication of how good a stayer a horse is, so it should be applicable to National Hunt.
However, I would strongly doubt whether there is a high enough reading in many NH bred horses.
*Assuming you believe that dosage is valuable in ascertaining how good a horse will be at staying