Is it possible to award salary points for additional responsibility on a fixed-term basis?
There is nothing in the School Teachers' Pay and Conditions of Service document which says that you cannot do this. We must therefore presume that you can, though it is not customary to do so and I suspect the teachers' unions would take a dim view of it as a general practice.
It is, of course, quite normal in the case of teachers taking over posts on a temporary basis, because of illness, maternity leave or secondment. What is important, in order to avoid any misunderstanding, is that the nature of the appointment and the time limitation are very clearly set out in writing.