Who among our foes even does prolonged sieges? The skaven, I suppose, but they'd be doing it underground or inside mountains, where our ship would be of little use. But greenskins? Their idea of a siege is a hasty assault en masse, where they bring overwhelming numerical superiority and a broad willingness to charge into suicidal odds to bear against fortified positions. The most they ever did was use sappers en masse to conduct a same-day assault on a citadel. Beastmen? They lack the patience, equipment, or expertise for a deliberate siege. Norscans? Sure, but they'd be unlikely to cross the sea to conduct a deliberate siege against the Empire when their MO is coastal raiding. Chaos? If Chaos is laying siege to a city, the entire Empire is marching to relieve it, minimum.
Realistically, even an army of orcs would fail when making hasty assaults against fortified cities, because walls don't care about your willingness to charge to certain death, and ladders leave you horrendously vulnerable to getting stabbed in the head by a spear as you climb up or shot by arrows/bolts long before you reach the top. Crude catapults aren't nearly enough to knock down a well-built wall and lack the range to safely attack walls that have their own long-range defenses. Their best bet is sappers once again, because the digging speed of evil factions in this setting is beyond absurd and simple things like what happens to the dirt and rock you dig through can be ignored by them by pure magic, somehow.
A single flying ship coming in to help would be of great use in wrecking the siege engines the attackers would be trying to use, as well as attacking the besieger's war camp with stuff like fireballs, but in terms of delivering reinforcements, I have to say that short of bringing in a ship full of hero units, it won't make a significant difference. It's the ship's firepower that would be the key.