There were no nominal activities for the middeck crew related to flying the vehicle. There were no switches or controls on the middeck accessible to seated crewmembers in the ascent/entry seats. The rectangular objects in the photo are middeck lockers which are located external to the avionics bays.
Mission Specialist 3 (MS-3) did have responsibility over side hatch jettison, an action that would be taken only in the case of the Orbiter not being able to reach a runway, and which was never taken in the real world. MS-3 would also extend the escape pole through the hatch opening, after which the crew would use it to bail out.
Accordingly, there are two nominal callouts for MS-3 in the Entry Checklist, both related to the side hatch:
- Just before the deorbit burn, MS-3 unsafes the side hatch in case jettison is needed.
Remove and stow Side Hatch UV Filter and Locking Device, and Pyro Box
- After wheel stop, MS-3 safes the side hatch, since jettison is no longer required.
Ck with MS3. Report to MCC: “RJDs OFF, SIDE HATCH, DRAG CHUTE, AND
LANDING GEAR SAFED” (MCC report to NCC)
That is it for the nominal case. The timeline below shows the
MS-3 actions for jettisoning the side hatch and deploying the escape pole during an Orbiter bailout contingency case.
MS-3 had one instrument visible to them, an pressure-operated altimeter mounted on the locker immediately in front of them. This allowed them to monitor the progress of the cabin being vented to ambient, an action that was taken prior to side hatch jettison.
(Images credit NASA, annotation and editing mine)
Source 1: Tom Jones described ascent and entry on the middeck in Chapter 7 of his memoir Sky Walking.
Source 2: Generic Entry Checklist
Source 3: Crew Escape Systems handbook
Middeck image source: Shuttle Crew Operations Manual