I'd actually recommend instead going a bit further since it'll be more reliable and easier to setup on the client side. Use the iSCSI gateway and an RBD image instead. This'll get you the availability of Ceph and is much better supported than using FUSE or s3 for booting. You can even install windows on an iSCSI target and PXE boot it (disclaimer: i've only read about this being done, not actually done it) so that you don't need any local storage at all on the remote machine.

You'll still want a fast network (I'd recommend 10gbe on the server at least, 2.5gbe on clients though 1gbe will work you will notice it bottleneck in bursts) but that won't be any different than any other network booting/rooting process

https://docs.ceph.com/en/latest/rbd/iscsi-overview/

If you're going that way, you'll be much, much happier with RBD.

https://docs.ceph.com/en/reef/rbd/

By “I want to boot off this thing” I mean that, if I have a computer with three disks, I want to run a normal-ish distro on that computer with those three disks possibly as /home or /var or maybe even as /.

I’m sure it’s possible. I don’t think this is quite what Ceph is intended for :)