Lumley, who was last on stage in The Lion in Winter in 2011, and Scott, whose recent credits include The Emperor and Galilean (National), Cock (Royal Court) and Sherlock (BBC), will appear in Family Voices and Victoria Station.
The productions will be staged on the set of The Hothouse from 27 to 29 June, directed by Trafalgar Transformed associate director Edward Stambollouian.
Other events include a Pinter masterclass with Jamie Lloyd (17 June) and panel discussions featuring Lloyd alongside Pinter's widow Lady Antonia Fraser and his biographer Michael Billington.
First performed at the Hampstead Theatre in 1980, The Hothouse is set on Christmas Day in a nameless state-run mental institution where the inmates are subjected to a tirade of mindless cruelty. A maniacal and self-obsessed leader breeds a contagion of hierarchical savagery amongst his staff, who thrive on a noxious diet of delusion and deceit.
It runs at Trafalgar Studios - which has been reconfigured in-the-round for Lloyd's season - from tonight (9 May, previews from 4 May) until 3 August.