I remember seeing patches for marking channels as identical in terms of EPG (so e.g., a DVB-S version of the channel can automatically share the EPG entries from the DVB-T version), but has anyone worked on a patch for identifying simulcasting channels, or even had some preliminary design thoughts for how one could be implemented?

I can imagine at least the following two use cases:

1) When watching live TV, channel '1' automatically selects the 'best' available version of 'BBC One London' depending on available tuners - e.g., rotating through BBC One HD (DVB-S2), BBC One (DVB-T2), BBC One (DVB-S) and BBC One London (DVB-T).  This is much more intuitive than having to manually find an available broadcast of the channel.

2) For recordings, if a timer needs to start a recording on 'BBC One London', depending on its priority and a 'required quality' attribute it will similarly locate a tuner to make the recording with. Assuming it needs the DVB-S2 tuner for HD, if someone with lower priority is currently watching live TV on BBC One HD via DVB-S2 (and only one HD tuner is available) then they will be transparently switched down to the next available tuner quality.