Is "offloading a passenger" idiomatic?
Solution 1:
Offload is used in the cases you mention to distinguish the action taken from the more commonly used disembark.
Disembark is the normal word used for people getting off a conveyance (originally ships, but extended to airplanes, trains, buses etc.) but using disembark is a little bit ambiguous.
Jane Doe was disembarked from the 9am flight this morning
Jane Doe was offloaded from the 9am flight this morning
They both mean the same thing, but as it's normal for people to be disembarked in a non-forceful way, using offloaded for a passenger makes it clearer that the disembarkation was against her will rather than the normal procedure.
OED does mention that off-load has been used for people.
off-load v. orig. S. Africa (after Du[tch] afladen)
trans. To unload. Also transf. and fig., to discard, get rid of, relieve oneself of (a person or thing). Hence off-ˈloading vbl. n. and ppl. a.1952 C. Day Lewis tr. Virgil's Aeneid v. 113 They enrolled the women for the colony, off-loaded the men who wanted To stay there.
Also compare the definitions from The Free Dictionary and note defn.3 for offload.
Disembark
v. dis·em·barked, dis·em·bark·ing, dis·em·barks
v.intr.
1. To go ashore from a ship.
2. To leave a vehicle or aircraft.
v.tr.
To take ashore from a ship.
http://www.thefreedictionary.com/disembarkOffload
v. off·load·ed, off·load·ing, off·loads
v.tr.
1. To unload (a vehicle or container).
2. Computers To transfer (data) to a peripheral device.
3. Informal To get rid of and pass on to another
v.intr.
To unload a vehicle or container.
http://www.thefreedictionary.com/offload
Solution 2:
The OED has some examples of humans being offloaded e.g.:
1968 M. Woodhouse Rock Baby v. 43 A Director who has to offload one of his staff and is embarrassed. and,
2001 FourFourTwo Sept. 104/3 Lazio were prepared to offload Juan Sebastian Veron because they had secured the services of Italian playmaker Stefano Fiore.
But the OED doesn't have any examples of persons being offloaded from public transport vehicles.
I would nonetheless adjudge offload an appropriate idiomatic use, though it does carry a very slight implication of it being done against the passenger's will. (Though I can well imagine that in internal airline jargon they talk about offloading passengers in regular context.) But disembark would be more polite when speaking to the public, and points more to it being at the passenger's request or convenience