It's obviously not "legal", as per the Ars article you posted (thanks for the interesting read BTW) these Google Play components are proprietary closed-source apps, that are only available under a license to the Open Handset Alliance members, which Jolla is of course not, since it's manufacturing a non-Google approved version of Android. So while you might be able to install Play on your Jolla, I would think its impossible for Jolla to ship their phone with Google Play pre-installed. Though depending on other restrictions on the phone, it might be possible to make an easily downloadable "pirate" package that will install unlicensed Google Play on the phone, so that non-technical users can benefit from the full Android ecosystem.
What comes to Google "blocking" this hack, I am quite skeptical. If they add some software detection that you are running the Play services on a licensed device, we know from PC world that it will be hacked and people will start distributing "cracked" copies of the APK (Android application package). But AFAIK, Android phones support DRM as well, which can make things little tricker if Google one day really wants to shut off this competition.