Yip, we probably could have done it as a "slave 168" shield. But didn't, because this design in total uses less components than say, an NG - and can be as small as two RBBB's end to end.
A shield also wouldn't allow certain components to be shared, like the resonator - although, two 168's as a shield would.
But I'm not sure how you'd program the chips if it was a shield, I think you'd still need the mini-USB adapter - if anyone thinks up a way that this can be a shield in the next 24 hours, then let me know, we've got a hundred pcb's on order that won't go into production for another day or so.