Is this using some kernel feature or similar? I'm curious why this claims repeatedly that it only works on GNU/Linux: does it have dependencies both to the Linux kernel and GNU userspace tools that can't be replicated on BSDs or other userspaces?
I'm also interested in why this requires root access, though that answer may be related to the first.
As far as I'm aware from my brief use of tomb, it's a small shell wrapper around GPG and dm-crypt[1].
I suppose realistically anywhere you have access to the required tools (or something that pretends to be them but does platform-specific stuff underneath) you could use tomb.
I'm also interested in why this requires root access, though that answer may be related to the first.