Somewhere in the game files has to be the Base ID for these armors and items without their enchantments, but I haven't seen a single thing. I keep checking mods without much luck. But for some strange reason I can bet if someone did find the Base ID, they probably would not be recognized as enchantable or upgradable without a crafting+ mod.
Keeping my fingers crossed X
edit-- by the way, there are mods to add extra enchants to the pre-enchanted game armor. I'm assuming you're asking for a non-enchanted version of Nightingale armor so you can apply your own enchants, of which I have not found.
[Edited by OffCentered, 12/15/2011 6:19:50 PM]