However, it was far too easy to activate accidentally. I kept interrupting my own work because I'd unknowingly pushed the mouse over to that corner, and would have to stop what I was doing to wait for the password entry box to come up. Annoying.
Then I realised that Quicksilver could come to the rescue again (Quicksilver Ho!^1^). I removed the Hot Corner and set up a trigger with the text:
open '/System/Library/Frameworks/ScreenSaver.framework /Resources/ScreenSaverEngine.app'
as the item and 'Run command in shell' as the target. Then I assigned a mouse trigger to the bottom left corner, but crucially required a click in that corner to activate it. It's nearly as convenient as just putting the mouse in that spot, but not nearly as easy to trigger accidentally. Sorted!
^1^ For some reason, I imagine a cowboy in a silver suit and a black mask calling out "Quicksilver, Ho!" before he gallops off on his horse to rescue someone. ↑