So I kind of 're-wired' it using the free Cockos plugin - Reastream (part of the Reaplugs bundle). Midi goes out from Mulab into Orion using a virtual Loopmidi port (another freebie, 64bit only I think)
The live audio stream comes back in to Mulab via Reastream to be recorded or whatever.
It's a bit fiddly, as I had to set Orion to use a different soundcard (e.g. internal computer speakers) and mute it. But it's stable with little latency and integrates well into the new DAW without need for exporting audio.
Anyone else doing something like this? I guess it's like Rewire, but a bit fiddlier.
I guess it would work in other DAWS too. I don't know if you can 'sync' the DAWs though, that would be great.