According to this there are four Voyagers. Those plus the HSTs could provide a reasonable main line service. They could also be used to Paignton.
One train plan I have seen appears to suggest that they will try to operate trains approximately in the normal time slots on the main line with the exception of a couple of services. The branches, too, will use the normal timetable. This, I think, avoids the time-consuming process of drawing up a new timetable (which would need NR approval), retains branch connections and is less difficult to explain to the public.
I doubt, though, whether all branch services will have their full number of coaches.
At present, NR are expecting to reopen Whiteball on Monday morning (10th February 2014) as planned. Until then, fast coaches will operate direct from Plymouth to Taunton, taking about 80 minutes. Once Whiteball is open the coaches will go to Exeter in slightly less time. Because the rail journey is normally quite slow, the overall journey time between Cornwall/Plymouth and Paddington (and other destinations) will not be too greatly extended.
Congratulations to Network Rail and First Great Western at very difficult times.
Many thanks for this snippet of information - we must stress that this is not an official report.