This is probably
ン instead of
ワンマン is wasei-eigo from "one man". For example,
ワンマンショー (one man show), or
ワンマンライブ (meaning a live performance with just one band, not necessarily one person).
In the context of buses,
ワンマン means that a bus is being operated by a driver alone, with no conductor (i.e. every bus nowadays). It used to be a rule to have this sign if there was no conductor.
So this is not a phrase ("one man doorway"), but a two-purpose sign showing the doorway and stating that the bus is
Of course, this being FLCL it's possible that it actually does say
ワソマソ, as a joke. A Google search reveals that it is sometimes used jokingly.