I don't know of any featherweights that run castors [...]
The Honey Badger does use a castor on the front and previously did so on the rear when the wheels were located in the middle. Strictly speaking the rebuilt version doesn't need it., it has enough force to move on its HDPE base with the motors driving it along but I find it helps make it a little more controllable. I use castor balls in mine (http://www.ebay.co.uk/itm/301529661473) rather than a wheel to help with ground clearance.