This is already possible to do.
Select the Level Dimension Tool and hover your cursor over the Landing you want to dimension. The pre-selection highlight will indicate that the whole Stair will be dimensioned. This is not what you need, so press the TAB key to cycle through the possible items to be dimensioned, in this case, it will cycle between the Stair and the Tread, as indicated by the Info Tag. Click to place the dimension when the Tread is highlighted with the Pre-selection Highlight.