[jcynix]

That's what Dropbear is supposed to do, it can be included in the initramfs of a server: https://matt.ucc.asn.au/dropbear/dropbear.html

Disclaimer: I still didn't try it myself yet, but it's on my (too long) list of things to do.

[gnufx]

[I don't think it has to be dropbear specifically.] https://nixos.wiki/wiki/Remote_LUKS_Unlocking has a recipe also including Tor. You still have to be present to unlock the system, unlike with Mandos -- which I wish I'd been able to set up for our office, even before the Great Work From Home.