[I’ll answer this in text because I’ve been unexpectedly super busy this past quarter and there were actually several versions of this question being asked.
Persona Q takes place during the Yasogami school festival in P4 and in September in P3 (if I recall that was when their school festival was cancelled due to the storm and Makoto/Minato/P3MC gets sick for three days). The P3 cast gets transported through time and space via the Velvet Room to appear in Inaba to the time of the events in P4.
Labrys was still “asleep” back then. In P4U, she gets reactivated during Golden Week after P4’s True End. They’re gonna have to do some major reality shifting to get her to be playable. At the most, expect a reference or something from Aigis if anyone happens to ask about her being the 7th generation.
The most we could hope for is unlockable playable characters of Labrys (or Metis or FeMC even) that doesn’t affect the main story but are just neat little
fanservice bonuses for the player.]