To Answer the original question there is no access to the cargo bays on the 737. The freighter conversions vary in design depending on who completed the mod. AEI, IAI and PEMCO all have blow-out panels either side of the 9G bulkhead but they are not entry points to anywhere and only designed to equalise pressure either side of the bulkhead. I have not seen the 737-800 conversion yet but I can't imagine it to be any different.