I really do hope you are only writing a novel, and not planning anything more sinister! This smells of a rendition scenario...
The hold is large enough to accommodate maybe three people sitting, arms behind their backs and legs in front of them. Two would be more 'comfortable', if you care about that with your victims. With crew/pax luggage in there as well, it would be crowded.
The baggage compartment is the aft most pressurised part of the fuselage. It does not have independent environmental control - to be honest, I cannot recall whether it gets hot or cold, but I would guess that it gets cold after a while. It is accessible by a sliding door from the cabin, via the aft lavatory. Pax and crew are able to access baggage inflight, if necessary. The sliding door separating lav and baggage compartment is a thick and heavy one, designed to act as a secondary pressure bulkhead in certain circumstances. Most importantly, it can maintain the cabin pressure vessel if (say), a loose turbine blade punches a hole in the fuselage at the baggage compartment. You are also able to (remotely) depressurise the external baggage compartment door seal, allowing air in the compartment to escape for smoke evacuation (or prisoner suffocation). There is a rotary handle above the baggage compartment access door which allows you to control this depressurisation from the relative safety of the aft lavatory.
Note that because the door is a secondary pressure bulkhead, there is a limitation on leaving it open for more than 5 minutes at cruising altitudes. The door position is monitored in the cockpit via the Central Warning System when flying above 40,000' - opening the door activates the blue advisory caption "Internal Baggage Door". After 5 minutes of being open, this advisory goes amber, as an aircraft limitation has been exceeded. Note that if your prisoners break free and try to get out, the cockpit would be immediately alerted to the door being opened... Flying below 40K, these indications are inhibited.
The baggage compartment is also accessible externally, for loading. This door is operable from both the inside and out, but it is possible to lock this door from the outside to prevent intrusion. I am unsure whether this also prevents escape - does anyone out there know?
If you are not writing a story and you are actually researching your options for something unsavoury, I would appreciate you not mentioning my assistance if you get caught.
Cheers