In a nutshell, I'd prefer to keep the narrow no-break space.
Some French styles uses "normal" spaces or, better, normal no-break spaces, but much more by ignorance than by choice.

(And Frank' proposal of a Processor option w/Propachi seems a good option to me)

We're not going to want to change this in citeproc-js and we'll definitely not make it configurable in CSL. Frank may be willing to put out a propachi build, but he's been busy and I obviously can't make promises on his time.