I know Kings Cross (possibly not platform 0) is long enough for an extra carriage, but would there be any stations where the length would have been problematic? Loading my bike at Peterborough into the DVT seemed pretty damn close to the ramp, but I wasn't paying much attention really.
As it is, I feel that with the Mallard refurbishment programme it is a real shame that they didn't (as far as possible) standardise the layout of the two EC fleets, to have the same 1st/Standard tratio, same number and numbers of bookable seats, tables in each carriage etc so that (for example) if sets are substituted the reservations would simply transfer easily- with any extra seats that appear say in coach B of a mark 4 set but not in the HST (because of the guard's office) being non-reservable.