Can the Bugaboo Cameleon be used from birth using the seat?
No, not really. The seat is shaped and a newborn needs firm, level surface for longer periods of time. In theory, you could be using only the seat IF you got a V-shaped insert, something leveling the seat - e.g. like the Quinny Newborn-Insert (From-Birth Cocoon) which does that.
If you were putting a newborn in such a bucket seat without it, you would be damaging their neck and spine and obstructing their breathing.