Hi all. I’ve just figured this out and since there was some quirks to the solution that weren’t obvious I thought I’d just highlight the process here in case anyone else is interested and currently baffled. 
I’m assuming you’ve already got a DI track in your project. I came at this from having previously recorded guitar tracks and DIs simultaneously.
Setting up Kemper
- Ensure SPDIF Out on MR816 is connected to SPDIF In on KPA.
- Ensure SPDIF In on MR816 is connected to SPDIF Out on KPA.
- Press and hold Input on KPA and select SPDIF Input Reamp as Input Source.
- Press and hold Output and make a selection for SPDIF Output; I chose Git / Stack.
- Go forward a few pages and ensure SPDIF Clock is set to you project sample rate. This option is for OS 5.5+ only.
Setting up MR816 and Cubase 8 Pro
- Before opening Cubase, open MR Editor as you’ll need to turn up the SPDIF channel levels and you can’t seem to do that in Cubase.
- In the setup make sure Digital IO / External FX is set to SPDIF coax. Remember External FX is for the internal FX!
- Select the SPDIF coax tab on the right and the DAW 11-12 channel strip should highlight
- Turn the up the blue cheese control from off to 0.00. I also turned up the fader too. Not 100% sure if both these things are required but I assumed one was In and the other was Out.
- Start up Cubase and click Devices > Device Setup.
- Ensure that the SPDIF connections are shown in the IO list and marked as Visible.
- If not, click on the Control Panel button and set Digital IO again as above. It might reset if you’re already using the MR816 DSP FX on the project. If that’s the case, you’ll need to put that back once you’ve rendered your reamp as you can’t do both unfortunately.
- Exit the setup window and click Devices > VST Connections > External FX tab.
- Add External FX Bus and select the SPDIF Coax as the connection in and out. You should be able to set this as mono as you can set up your KPA to route both the DI and full amp sounds separately on the left and right channels. Label it something sensible like Kemper Reamp.
- Add FX Track and for the plugin select External FX > Kemper Reamp
- Route your DI track directly to the Reamp FX track.
- Now, if I haven’t forgotten anything, you should be able to press play and bask in your reampy goodness.
- That’s about as far as I’ve got but my understanding is that once you’re happy with your profile you do File > Export > Audio Mixdown and batch export with just the DI/FX channels and it’ll render a newly recorded guitar for you.
- Assuming you don’t have any latency issues you can also do this neat technique for comparing multiple profiles: Set up a track for recording the standard analogue output; set markers and loop over the section; hit record and change profile manually on the KPA every time you hit the end of the loop. This way you just need to expand the track lanes and you can easily preview different profiles for comparison.
So there you go. Pretty neat technology!