It's an interesting question. I think you'd need at least:

  * a way to define scheduling requirements on a process level (beyond current, coarse, priority settings),
  * bounded process message queues (this would be handy in soft-realtime Erlang),
  * deadline accurate receives, 
  * accurate control over resource allocation

I've been meaning to sit down with some theory and think through this more analytically, but the above are my BART thinking-time derived set of requirements. I'm sure there are cases--some which would be obvious, in retrospect--that are not covered by the above.

You'd have to run erlang directly on the hardware without an intermediary OS and you'd have to schedule the individual erlang processes using pre-emption and by giving them priorities. You'd also have to add a ready-list per priority.

After all, as long as BEAM is running as a child process of a host os that is not hard realtime you can't guarantee much of anything.

Well Erlang is running on RTEMS in production in industrial control systems http://www.grisp.org

Hard realtime processes to come

Of course. Apologies, I assumed that was a given. A large part of the appeal of Erlang the Language is Erlang the VM and the two aren't often divorced.

Yeah, that's what I meant with LING (Erlang on xen).

I have a project planned which when funded will give us hard realtime guarantees for certain Erlang processes.

This can only work of course if the underlying OS is also hard realtime capable. I have ported Erlang to the open source RTEMS http://www.rtems.org already. More details under http://www.grisp.org (sorry website not up to date but soon will be).