Has the evolution of feathers *subsequent* to the origin of flight been proposed? Er, since the days when pterosaurs were thought to be the ancestors of birds, that is?
Yes. The assumption that feathers in general are "for flight" is a fairly integral part of BAND (see Feduccia's books) and was rather widespread even outside it till *Sinosauropteryx* was discovered.
The stage I and II feathers on *S.* do not make sense unless feathers evolved before flight or *S.* is secondarily flightless. Judging from its skeleton, the latter hypothesis is a lot less parsimonious than the former.