My concern is that if you are signing in, there seems to be no way to turn on magnifier unless it was 1) previously turned on or 2) you navigate the tiny status bar that is too small for a low vision user to navigate without magnification.

Here's an example of the issue: 
# Enable magnifier in settings while signed in
# use ctrl + alt + brightness up to magnify 
# tab and mouse around, everything is fine
# sign out
# on the sign in screen, pressing ctrl + alt + brightness up does nothing

Sign in screen AND log in screen: magnifier can be checked and turned on via status bar > accessibility > screen magnifier. Once it is on, I can sign out and in and it is on by default in both cases. I can magnify using ctrl alt brightness up or down. 

Laura A's comments:

I don't believe there is a way to do this, so it sounds like this would be a feature request to have a keyboard shortcut to enable the fullscreen magnifier from any screen, at any time (similar to Ctrl Alt Z for ChromeVox and Ctrl Search H for high contrast). Maybe we can explore having another Ctrl Search plus *something* shortcut? I know we don't want to add a huge amount of shortcut toggles, but I do think this one makes sense, especially for anyone who simply can't see the settings at their current size and needs magnification to be able to navigate.