Then again, the Arcane Age have some very specific rules about time travel, and a ninth level spell to make limited 1-year trips. And then, Mystra, Azuth, Oghma, Deneir, and Labelas Enoreth collectively watch out for time travelers. And Labelas' Knights Paradoxical are more or less Faerûn's "Time Police". That being said, you definitely could find out information on the Imaskarcana in the past; you just couldn't physically take it (or any magic item) back to the present with you.
For me, the "rule" in arcane age was a way to try to make characters in one time setting fit within all the parameters of Netheril - i.e. we don't want a wand of magic missiles going back to before the Netherese general invented it. Silly if you ask me, especially considering the lengths TSR went to blend the worlds in to Planescape and Spelljammer settings, you could go back in time in Krynnspace, hop a Man-O-War and be on Toril any time.
Also, the original Old Empires accessory had the Font of Time in Mulhorand (which is right next to Imaskar, by the way) which allowed time travel under an entirely different set of rules.
And there's an astronomical feature (a constellation, as I recall) in Greyspace that can toss spelljamming vessels to random points in time and space.
I've long held the (personal) view that there are lots of Imaskarna - the term originally being used in Imaskar to denote a magic or lore repository. There are the "offical" Imaskarna (of which yes, we've only seen the "odd" ones - but in my view that is because the even ones are ... well, slightly more ... mobile and the seventh, ties them all together ...) and then the other Imaskarna made by various wizards and groups/cabals of that realm.
-- George Krashos
"Because only we, contrary to the barbarians, never count the enemy in battle." -- Aeschylus