My boss is the subject matter expert, and most of us are learning from him. He calls in employees for long meetings where he explains how to do a task, and, for this entire meeting, the employee must stand next to the boss while boss demonstrates task on the computer. Employee has no choice but to stand. This is extremely uncomfortable, precludes the possibility of easily taking notes (you have to hold your notebook in the air) while looking down at screen, and frankly, feels like being a kindergartner standing next to your teacher in a schoolroom (except a kindergartner is short, so at least they see eye to eye.) It is profoundly uncomfortable and in other companies, long duration instruction at a computer would take place with both persons sitting. Am I wrong to find this somewhat demeaning?
Talk to your manager
It's likely he doesn't consider the meeting to be long or understand firsthand that it's difficult to take notes. I recommend you talk to him one-on-one about how it is difficult to take notes standing up and if the meeting could be done in a place with adequate number of chairs for everyone. I think it'd be better to focus on the difficulty taking notes because it seems less likely your manager will to take offense to you bring this up.
Typically, you use a standing meeting to keep the meeting short, but if the meeting organizer is sitting that doesn't work anymore.
Before the manager demonstrates the task, ask if you can bring in a chair to sit an take notes. I know you said that this request will likely be treated with disdain and irritation but you won't know the answer if you never ask.
If the manager remains impatient and doesn't let you grab a chair, I would lay my notepad on the desk and lean on it for any note taking. Taking notes standing up for 20 minutes is just ridiculous. If the manager makes an issue of this I would apologize and then brush up my resume and look for a new opportunity as it would be clear that your manager is a jerk.