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.