The top level, where the Drifter is, can be accessed with a bit of effort.
There is a skeleton of a man behind an overturned pew that he appeared to be using as cover, as well as 2 ammunition boxes and a combat shotgun next to him.
There is no need to climb up into the rafters to search the drifter if you can reach any part of his body hanging down. If you can't reach him, you can use an explosive such as a frag grenade to knock him down.
Dickerson Tabernacle Chapel only appears in Fallout 3.