I've tried retroactively mounting it using
mount -t hfplus
but I get a 'wrong fs type' error.
Is it yet possible to mount APFS without commercial software?
No, and it probably won't be possible for a long while. From Apple's APFS FAQ:
Is APFS open source?
An open source implementation is not available at this time. Apple plans to document and publish the APFS volume format specification.
So after Apple releases the APFS specs (at some unspecified date), someone will have to do the implementation work, and it would probably take many years before it attains a decent level of functionality (especially if you want write access).