I spent ages last night trying to scan for satellite channels from within Myth, but no luck. In the end I did a full scan from the command line using dvb-utils:
scan /usr/share/doc/dvb-utils/examples/scan/dvb-s/Astra-28.2E | tee channels.conf
…and imported the resulting channels.conf file into mythtv-setup. To my surprise, it worked! I navigated to “Watch TV” in mythfrontend and suddenly had access to hundreds of channels of complete drivel. The EPG even worked.
Next step: HD.