Three Enterprises have had people traipsing inside supposed nacelles:
NX-01 in ENT "Catwalk"; the engines had to be shut down first.
NCC-1701 in TAS "One of Our Planets Is Missing"; the engines had died first.
NCC-1701-D in TNG "Eye of the beholder"; only a small control cabin, but the engines were in varying states of activity during the adventure.
Timo Saloniemi
According to the newer TNG tech book, the nacelle control room is actually located (rather interestingly) at the rear of the nacelle, and is accessible from a Jeffries Tube (which I infer to in turn connect to the nacelle pylon and the engineering hull).