A herald on foot is just an infantry model, on a palanquin... it is also still infantry (uless that has been errata'd... just checked the BRB and it has Troop type infanty :O
But the only shooting attack that can pick a character out from a unit (that has more than 5 rank and file models) is Sniper. if there are 4 or less rnf models, then you can put hits on the herald, otherwise... no dice.
(page 99 for shooting concerning character in unit [under heading shooting])
(page 488 for palanquin being infanty)