The solution, not only to this specific audio interface but any sound card not properly communicating this to PulseAudio, was to use JACK via QjackCtl with the following script (needs adaption to your sound card and channel configuration):
#! /bin/bash
pacmd load-module module-jack-source channels=1; pacmd load-module module-jack-sink channels=6 sink_name=firefaceout client_name=firefaceout channel_map=front-left,front-right,front-center,lfe,rear-left,rear-right;pacmd 'update-sink-proplist firefaceout device.description="RME Fireface 802 5.1" '
Place this somewhere you like, make it executable and add it to QjackCtl as a post-start script under “Settings->Options”. This will create a 5.1 sink in PulseAudio that routes all your normal audio software’s output into JACK and from there to your sound card’s output.
The only other thing I had to configure in JACK was the input and output device under Settings->Advanced.
You can add QjackCtl to your session’s autostart so it’ll be there as soon as you log in. There’s also an option to start it minimized and to start the JACK server automatically upon starting QjackCtl, so you won’t have to manually interfere with it after setting it up.