I've always kept my iPads locked in landscape mode, and to date, the OS has ignored the setting until it has finished booting, even on the models where it could be controlled by the physical side switch.
Granted, it's a software-configured switch (that can also be assigned to Mute), but it's not something Apple has chosen to alter in the boot sequence until now, apparently.
Even so, it will be interesting to see whether this applies only when attached to a Magic Keyboard, as alluded to above, or universally. Don't the Mac startup key commands work reliably only with Apple's BT keyboards, or a certain subset, after they effected that change to accommodate startup keys with wireless keyboards?
