I believe the problem isn't so much that DO, etc., don't support unikernels, as that they use something other than Xen as the virtualization layer; something-other-than-Xen is still on the to-do list of the unikernel projects.
The rump kernel unikernel (rumprun) will happily work on actual hardware. It's a bit hazy if that's a unikernel or an embedded system in that case (and what's the difference anyway?), but to avoid hairsplitting, you can replace the hardware with KVM and off you go.