And even getting waitlisted for a battlefield upgrade is hit-or-miss. Some gate agents won't do it for you if you have a companion.
ETA: one tip is to try to get the agent at the check in counter to do it, and if they won't then ask the gate agent. That way you get two bites at the apple.
I've had an agent refuse because "AS elites don't get to upgrade a companion on AA, so we don't allow it either." In fact, AS elites do indeed get to upgrade a companion:
https://www.alaskaair.com/content/mi...elite-benefits