I have modified my Breeze theme for SDDM to display a clock with a larger, thin monospaced font (see below). I had to do a bit of tweaking of the Clock object that Plasma provides, but now it works great. (For anyone wondering, the proportional font is Inter and the monospace font is Roboto Mono)
How can I extend these modifications to the Plasma lock screen? In my case, it still shows the clock in the smaller, bolder, proportional font.