Genghis,
From what I've heard, access to cockpits has H&S implications these days due to the (miniscule) risk of radiation from the radon paint in the instrument dials. Most museums seem to be of the unofficial opinion that this ruling is rather silly, as we all suffer larger amounts of radiation from our tv sets. Still, good old H&S insist that lines should be toed, so there you go.
However, for people who are serious about gaining access to the aircraft interiors for research purposes, there are usually ways of doing so. Usually best to contact the museums before travelling and arranging some supervised access; we must bear in mind that museum airframes are no longer working airframes but now historic artifacts and must be preserved as such, hence the apparent bar to letting the general public clamber all over them.
Hope that helps.
BeauMan