I found a promising looking project in the Lazarus examples called FontEnumeration, but when I ran it, it showed all 567 fonts on my machine but wasn't able to filter them when I selected FIXED_PITCH or MONO_PITCH from the Pitch combo and pressed "apply filter".
I've just tried setting the font of a displayed control to 'Courier New' and tried reading the Pitch and IsMonoSpace properties, and neither are set correctly. I'm forced to conclude that these properties are not supported with the current default widget set on the Apple Mac. That, in turn, has led me to discover another Lazarus feature, namely the Padlock icon on certain properties in the Ctrl-Space dropdown and the Restricted tab in the Object Inspector!
This probably explains why Lazarus does not attempt to limit the choice of editor fonts in the Options -> Editor -> Display dialog. I think I'm going to be forced to keep a list of all known Monospaced fonts and filter on the basis of that. Just the thought of doing that makes me feel dirty, somehow!