I don't think there is such access on the 737 - whenever we had to go into the electronics bay on the 737 or 757, we came in from outside.
Widebodies - 747/767/777 all have an access panel just aft of the flightdeck under some carpet in first class (obviously lower deck on the 747). I assume the 787 is the same but no first hand experience.
Getting from the E-bay to the cargo bay is a different matter...