Given this caveat, this seems unusable for production systems.
I'm guessing that "backups" would actually have to be live replicas set up from the start, and if the master fails, you convert a replica to master.
In addition, you could perform actual static backups by pausing a replica, backing up the actual table files themselves, then resuming the replica (and it will catch up). In case of total failure, you just dump the table files into a fresh install of MariaDB. (Copying database files is a common technique for migrating data, not just SQL command import/export.)
Is there any reason why these wouldn't work?
From the description it looks like it would be easy to do backups, it's just that mysqldump is not currently aware of temporal tables.
Just use
SELECT * FROM t FOR SYSTEM_TIME ALL;
And export it in an appropriate format.
They really need to create a workaround for that, because sooner or later you will need to migrate your data and right now it's simply impossible.
Edit: Particularly as, reading further, you cannot ALTER a temporal table. Which does make sense, but the problem remains.