That's a neat trick. The only potential problem is that multi-city itineraries on Google flights are very limited. Certain routes or options (e.g. extra stopover) just result in a message saying that no flights are available, while the AF/KL websites are able to price those. I wonder though if you could basically repeat this trick by taking the URL of a working example and modifying the URL parameters to whatever flight options/dates/routes you need..

Edit: Probably possible, however the URL contains the fare class as well as the exact fare code for each flight. If you have a way to determine those beforehand somehow it looks like you could, theoretically, construct a URL to get exactly the flight options you'd want.