Thanks for the replies.
Nah, font chooser does not allow sorting of the dialog, only filtering.
Maybe it's possible to simply replace there the `Family` box with 2 boxes, one containing monospace families, the other proportional?
I personally have no idea how GTK dialogs are programmed ( I only know a bit of programming) , but on my part, I could do the job of actually classifying all the fonts into those 2 categories.