I too would like to see RCD/GFI protection split to a per-circuit basis, although as you hinted, with the cost of combined breakers as they stand at present it would get very expensive.
I tried using single way RCBO's (with the neutral piggy back wire) and apart from the high price had little success with them actually working.
Now use double pole RCDs mounted in a 2/3 way DIN box (one for an MCB if needed) as they are only UK £25 odd with the box.
