It works for me. This is what I added to
kobo_extra.css
Code:
@font-face {font-family: monospace; font-weight: normal; font-style: normal; src: url("res:///fonts/normal/FreeMono")}
@font-face {font-family: monospace; font-weight: bold; font-style: normal; src: url("res:///fonts/bold/FreeMono")}
@font-face {font-family: monospace; font-weight: normal; font-style: italic; src: url("res:///fonts/italic/FreeMono")}
@font-face {font-family: monospace; font-weight: bold; font-style: italic; src: url("res:///fonts/bolditalic/FreeMono")}
Here's a screencap. I had one of my serif fonts selected in the Kobo font menu and
font-family:monospace assigned to the classes for these few paragraphs.