I don't believe that HPL ever addressed that, but we can speculate:
The Necronomicon had several editions:
- Original written some time before 738
- Translated to Greek in 950
- Translated from Greek to Latin by Olaus Wormius * in 1228
- Published in Latin in the XV century in Germany and in the XVII century in Spain
- Published in Greek in the XVI century in Italy
- Translated to English by John Dee in the XVI century
Over the years only the Latin version seems to survive, although Pickman had apparently the Greek copy, there was also a Greek copy that disappeared some time after the Salem Witch Trials.
In other words: it is rare, but not extremely rare, since so many different editions exist (yet indeed only 5 libraries have copies, including two versions at Miskatonic).
As to accessing it from the library on the Miskatonic University - I do believe that indeed, it would be not difficult for the faculty member (or someone with connections) to get access to it, since it would be just a "rare book".
From the information at the University of Adelaide:
Rare Books & Special Collections is open from 9am to 5pm weekdays to members of the University and to members of other institutions and the public with appropriate identification. Material is not available for loan and must be consulted in the Special Collections reading room.
Access to the manuscript and archival collections is by application. These collections are covered by formal access conditions and some collections and parts of collections have additional restrictions imposed by the donors.
*Ole Worm was born in 1588 so either HPL made mistake or we are talking about different person. Or Yog-Sothoth done some timey-wimey things with time