Were there any passages linking Boiler Rm #6 to the post office and/or cargo holds (motor car hold, baggage etc..)or was the fireman's passage the only means of entry into those forward-most compartments?
If you look at the plans here: http://dsc.discovery.com/convergence/titanic/explorer/explorer.html
access to the forward compartments was via the forward hatches. The firemen's passage gave access from where the firemen, etc were berthered to where they worked. They had no access to the hold areas. The Post Office was shut off from the rest of the ship. There were gates [kept locked for security reasons] that gave access to the 1st Class baggage areas. The clerks had access upto E-deck and from there into 1st Class.
The lower portion of hold #3 was a "bunker hold" designed to contain 1,092 tons of coal for use in the ship's boilers. Stokers and trimmers could access this fuel by going through the watertight door closing bulkhead D and into a small vestibule. From there, non-watertight "man doors" led into the bunker hold. These two doors meant that the so-called "black gang" did have easy access into the lowest portion of hold #3 directly from boiler room #6.
However, getting into hold #3 and accessing the mail or baggage storage rooms on the Orlop would have been another thing. It would have been necessary to scale the trunking of the bunker hatch and hang by fingernails while opening doors secured from the other side.
Looking at that small vestibule, the after watertight door sealed bulkhead D. A second watertight door (centered between the non-watertight bunker doors) actually served to seal bulkhead B. The fireman's tunnel was effectively an extension of bulkhead B pulled aft right through bulkhead C. There was no closure in the tunnel at bulkhead C.
So essentially, for Jack and Rose to do what they did those final minutes before the iceberg, They would have had to have entered Bunker Hatch #2 somehow, scale down, enter the Motor Cargo (somehow. I don't see a door exiting out on the plans) and then break open the crate holding Mr. Carter's precious Renault?