There are lots of discussions around the web about this. The long and short of it seems to be that you could do it in an rclone mount and it'll work well...until it doesn't.
I ended up just using rclone itself with --backup-dir but it is far from as good as Borg and the like for dated snapshots and pruning.
Out of curiosity, is there something about Borg that you won't get with restic? Restic supports many backends natively and then the rest through rclone. There is also Duplicacy which I hear good things about.
Answering my own question: I like that Borg supports rewriting to totally erase stuff that has been backed up while restic does not offer this.
I have also looked at borg-specific hosting and may consider it. But there is also something really nice about the simplicity, even if at the cost of efficiency and features, of rclone directly with the files.