It is a good way to rescue the unused space. Personally, I would go for the traditional way
mkfs.ext4 -m 0 /dev/mmcblk0p$n
mount -o gid=100000,uid=100000 /dev/mmcblk0p$n \
/home/defaultuser/extradata
or
mkfs.ext4 -m 0 /dev/mmcblk0p$n
mount /dev/mmcblk0p$n /usr/share/extradata
This is a straightforward way to go compared to the LVM approach. However, the LVM approach grants much better integration but less control because we do not know which partitions are saved or where data is saved.
Moreover, the LVM approach includes a tiny but not null risk of compromising the data in the rootfs or user home space. That risk increases when the user who approaches the LVM subsystem has no background as a UNIX system administrator.
While the straightforward approach presented above can only fail to deliver the expected result as long as the user does not format the wrong partition, obviously. Coding bugs and human mistakes cannot be thrown out of the scene completely.
Finally, adding an extraspace on your user home by mounting and a formatted unused partition includes the risk of leaking some sensitive data that should have been encrypted by default but is in the extraspace folder.
In my opinion, people in Jolla should have rescued that space as the default operation when the first boot after flashing took place. I may accept that I have overlooked some criticality about using the unused partitions by default.