Sapper: advancing, holding axe with both hands (something like this: https://encrypted-tbn2.gstatic.com/images?q=tbn:ANd9GcS25tmDCGjjVBfMVZz5AqU4UEXdbcxNKub977dAXJ4aJ0txW8zToA)
ADC: I'd much rather see this figure mounted than on foot (I prefer to have all my staff officers mounted), maybe pulling a rolled map/despatch from a pouch or reaching out to hand over said map/despatch to the general.
Officer: advancing with pistol; or alternatively, standing, sword sheathed, right hand raised, about to give the command to open fire.
Medic: kneeling or bending over a casualty.
Also, HaT, could you please clarify how the bonus figures will be distributed (i.e. how many sets must one buy in order to get each figure)?