You can still get a "paper" one day travelcard - but if you're going to be travelling before 0930 you will need a 'peak' day travelcard which costs more (but you'll get a receipt and be able to claim it back, so I guess this won't be a problem).

It's ironic that if you'll be making just two tube journeys on Oyster, this could be less than the price of a peak one day travelcard, but I can't see how you could get a receipt to show this... but if the client wants receipts, just buy the travelcard from a machine (you can get a receipt printed) or at the ticket office.

BTW, most of Docklands is Zone 2 or 3 - so you shouldn't need a zone 1-6 card.
(here's a PDF map of travelcard zones)