I can see the very top of the plunger's linkage assembly (it must have become detached from the trip lever linkage at some point, and sit's much lower in the drain now) and it has an "O" that I can get a hanger or hook around with some work.
The issue is that the plunger won't budge. It's almost as if it's lodged or rusted tight. When pulling with a coathanger bent into a hook, it takes me pulling so much that it unbends the hanger without moving. I don't feel any noticeable sign of give or wiggle.
Unfortunately, the drain opening is not an open drainpipe (it's some kind of closed stopper valve) so I can't try to do anything through there.
Is it likely that the plunger, itself, is jammed in there? Or, perhaps it's rusted? I considered throwing some DW-40 down there, or something to see if it might loosen. But, I figured I should ask first. I'd hate to open a wall, since it backs up to another shower, requiring me to demolish the siding to get access.