Maybe a progress bar looks better (and it more informative) in the lock screen. I think it would be user-friendly to change the display of the remaining time in the progress bar to something like this: 5 minutes -> 4 minutes -> 3 minutes -> 2 minutes -> 60 seconds -> --sec -> 1 second. This, in combination with a bar that changes every second, gives the user enough information without a busy/flickering screen.
While not exactly what you asked for, I just added something that gives you most of it. There is now a decreasing progress bar to the lock out screen. The text isn't replaced, and is still the same, but if you think (and I agree) that it flickers too much and that the bar is sufficient, you can check out the "display seconds" option in the plugin's properties.