Maarten Sjouw, since mds_backup and mds_restore do backup and restore binaries in addition to configuration databases, I can see how in prior to R80.20 described method would make sense:
Your restore would effectively perform an upgrade in the background creating new version branch and starting it from there on next boot.
It is the R80.20 underlying kernel and FS that make this path less plausible, since you are booting in to the version you are trying to substitute, so the file locks are possible, unless duplicate version of directory is being created as part of the restore process.
In this later case, there has to be enough logic baked into the mds_restore to account for it.
So I am looking for CP to let us know what the deal is and if there are specific caveats for R80.20 to R80.20 backup and restore.