You can do stream switching, in the same way as the Germans do it. Some might say, it's very 'clunky', and the picture mutes, and/or crashes about for a few seconds, while switching between the two versions which is (apparently) unacceptable.
But surely to do stream switching it still needs the transponder space for 12 linear programmes at certain odd times of the day. If they're going to do that then they may as well be there 24/7
You could do something clever with statmuxing - the regional services could go blank when there's no regional content, thus giving more space to the core service.
SVT do this in Sweden on satellite - they have PID switching opts as follows :
1. There are two core feeds of SVT1 Stockholm (I think) and SVT 2 Stockholm - which are the sustaining services.
2. There are Services for SVT1 and SVT2 for lots of regions. When there is only network content, these map to the Stockholm streams.
3. There are PIDs for a single regional audio and video service for each region. When SVT1 OR SVT2 opt-out, the reigonal versions of SVT1 OR SVT2 switch to their regional audio and video streams.
This allows opt-outs on both channels (but not at the same time) - and this is I believe the reason behind the PID switching (as it avoids simulcasting two regional services).
However there is a flash and a bang on the opt-out and opt-back - it would be impossible to cleanly do the regional idents and opt-out headlines/comings up this way, unless you soft-opted the whole junction and news bulletin (which would be feasible)
AIUI this is only used for SD regional opts - as all the streams need to be on the same transponder (re-tuning for an opt-out would take a LONG time)
So this isn't an immediate solution for HD...
(Sky aren't still doing the Anytime service where Sky+ boxes recorded a non EPG
channel to make on demand content available to non-network-connected boxes, are they? I'm trying to think of things which could be done with the leftover bandwidth during downtime...)
I think they are - for people who don't have broadband connections. Not sure it helps much though.