What's the easiest solution for MDM without a dedicated OSX Server?
Okay by now I figured out a way around the issue. My solution is to rebuild my FreeNAS server into an ESXi Hypervisor, which is capable of running OSX Server in a virtual machine. IMHO that creates quite a big overhead over running an MDM solution from a FreeBSD jail but it gives me a "native" Apple MDM, and the added ability to run other OSes in top of the ESXi aswell.

I see that this is not a generally applicable solution as it requires a server machine capable of running ESXi and a little bit of luck to get OSX to run on virtualized non-Mac hardware. But it works here so I mark this one answered.