Hi Tidalforce,
as far as I know every takeoff is allowed either with PACKS OFF, or APU ON + PACKS ON ... honestly can't see a good reason for an airline to inhibit the use of packs. Anyway for any PACKS OFF takeoff, the first PACK ON selection comes upon reaching Thrust Reduction Altitude, while the second at FLAPS UP/CLEAN CONF: as the two events never happen together, PACKS are always turned on individually, and consequentially passenger/crew comfort on pressurizing the aircraft is significantly improved.
Happy Takeoffs and indeed ... Unpressurized Landings !
fredgrav