Yes, you can specify canon AUs in your AU tags. However, so that the works are not just a continuation of something that is canonical, the AU tags must include some other change to the universe. IE: it's the mirrorverse but something specifically happens that didn't happen in the mirrorverse.
Examples:
Star Trek Original Timeline AU: Mirror Universe Timeline: Tribbles lead to the downfall of the klingons
Marvel Comics AU: The What If Where Magda Doesn't Die But It DOESN'T All Go To Shit For Magneto
DC Comics AU: Alternate Timeline Where Huntress Is Batman and Catwoman's Daughter But Mixed With The Normal Timeline
no subject
Examples:
Star Trek Original Timeline
AU: Mirror Universe Timeline: Tribbles lead to the downfall of the klingons
Marvel Comics
AU: The What If Where Magda Doesn't Die But It DOESN'T All Go To Shit For Magneto
DC Comics
AU: Alternate Timeline Where Huntress Is Batman and Catwoman's Daughter But Mixed With The Normal Timeline
NOT OKAY:
Marvel Comics
AU: 1602
Star Trek Original Timeline
AU: Mirror Universe
Buffy the Vampire Slayer
AU: Wishverse